Its some kind of weird thin leaf dutch hybrid. Soil looks too heavy on the peat side. The plant is big and expensive.
Need info on repot and watering .

Its gotten to that size without rotting in the same or similar soil - I wouldn't repot until it acclimates. But that is your call. I use cactus soil with added perlite (same recipe I use for everything but the amount of perlite changes depending upon the plant).

The part you are going to have a problem with is that Adeniums need hot sun to survive. Mine live on the west end of my Greenhouse as close to the ceiling as I could get them.

As Daisy says the trick with these is warmth and sun - if that one is from the Netherlands it must have been grown in a hot green house. I use pumice iso perlite but my soil mix is pretty much the same as Daisy's.

They are happiest when night time low temperatures are over ~17 C - not sure if that is something you get a lot. If they get below ~5 C at night consistently, these plants will start going dormant (indicated by the plants dropping their leaves) if you water them in these cold periods during their dormancy you will more likely than not rot and kill it. They are very touchy to cold and wet, more so than (m)any cacti.

Many of these (though I am not sure about your thin leaved version) are from regions that are hot and pretty dry but often with a surprising amount of rainfall, so when it is warm at night and hot during the day they actually can be watered way more than your average cactus, but I am not sure you would get hot enough during the day, say consistently >30 C for days/weeks at a time with hot bright sun on them, so I'd be careful with watering no matter what. Also make sure it does not end up sitting in any kind of standing water for any length of time.
So treat it with a lot of care, get it in the brightest light possible (but keep in mind that it probably grew in a green house, so even not very strong direct sun could still scorch/burn it) and then monitor soil wetness and night time temperatures and water with care.

Mark Dimmitt who is a long time cultivator and hybridizer of these based in Tucson, AZ, writes in his book that in the low desert of Arizona, for most of the year you can water Adeniums like you would a tropical plant in terms of watering frequency rather than a cactus, as long as the soil is well draining and quick drying. Once I started doing that I got my Adeniums to flower more consistently and longer, but we get lows >17 C at night for a large part of the year, when it is between 5 and 17 C at night I water them maybe once every 2 weeks, then when it gets below 5 C at night consistently (which does not always happen here) I stop watering them completely. This winter half of mine went dormant and the others which were located deeper under patio and closer to the house did not and they kept their leaves through the winter. We started getting consistently over 30 C for day time highs somewhere in late March and will not go down below it much or often until October/November, so I have been watering my plants quite heavily.

Here is one of mine with similar but not the same rather thin leaves that is currently putting on a flower show. It is an Adenium crispum hybrid that I got from Dimmitt when I attended one of his lectures on Adenium cultivation, not much of a caudex on this one, but since I upped the watering and regularly fertilize it with quarter strength orchid fertilizer it has become a much more consistent and floriferous bloomer:

Interestingly when it gets >45 C for a day time high on a consistent basis (which unfortunately happens here quite a bit as well in June - July), that actually turns out to be too hot for some of the Adeniums. I have an Adenium soccotranum that consistently goes dormant every summer and drops all its leaves. The crispum hybrid suffers through those periods, while my A. arabicum and my A. obesum hybrids take it with very little problem. Except that they all stop flowering.

OK, that last part about conditions here in Arizona are maybe not quite as useful for you, but it may provide some perspective on what these plants like.

Its a nursery plant. Its a grey area, since that same type of soils is unsuitable for cacti like echinopsis, lobivia, cleistocactu, parodia, rebutia, astrophytum, ferocactus, etc. I was thinking of adding more gravel, and a bit of coarse sand to the mix. My main doubt is the bonsai root system.
Heat isnt really a problem, ive read about the minimum growing temp- the room has steam heating..... its my germiantion room, where other euphorbia and apocyanacae i own live.
I couldnt lower it if i wanted to. Mainly why i asked about overwintering in particular - the cause of my complaints .Because that around 10 C" temp is unachievable because of the steam and electric heating in the flats where most of my plants are. Only one room(crassula and aloe) goes down to 15-ish, but humidity is too much for the desert cacti

I used to grow Adeniums in the Marshall Islands a few feet from the water. Our humidity was always above 70%. The Adeniums bloomed nearly non-stop.

It grows a lot in SE asia. Doesnt mean that our cold indoor humidity is the same as the hot outoor humidity where they grow in the ground.

For what it's worth, we experience an average humidity of 70-80%, but without the heat, and the Adeniums are pretty shy about flowering. I celebrate every bloom, put it that way. Certainly nothing like the plants in Thailand or places like that, where you can almost take the blooms for granted.

That's interesting, Baja. In the Marshalls our temps were about 86F. in the daytime, and 80F. at night year round.The Adeniums never went dormant and bloomed a lot. I know you've mentioned it before, but what are your average temps?

Depending on the season our high temps range from 68F/20C to 80F/26C, with nighttime temps below 68F for almost all the year and an annual maximum near 90F (maybe a few days a year over that number).

I'm envious! I'd love weather like that!

Clearly you're not an Adenium!

True!

I think if you want those plants (and some Pachypodiums, Dendrosicyos and the like) to grow anywhere near their full potential here, you have to put them in a hot house. That's the setup they use at the Huntington Gardens (in southern California, with a climate similar to ours but a bit warmer) and their collection is pretty immaculate.

Thank you Thijs for that really excellent introduction to Adenium cultivation. My own very non-expert advice about watering would be like Dimmitt's... these plants act like succulents for part of the year and tropical plants for the rest of the year. My own compensation for watering based on the season would range from twice a week in the summer to once a week in the winter, for a plant that's in full sun (direct sun for at least half the day) year round. The plant never actually goes totally dormant but it also does not go particularly leafy either. It broke the last pot it was in so it's definitely getting fatter. With actual heat you have to dial up the frequency in summer for best results, and similarly with actual cold you have to dial down the frequency in winter. I'm pretty sure I underwater in summer, and if we had actual summer rain the situation would look very different.

Kind of a typical mid-May look going on right now... and for comparison, the Pachypodiums growing nearby.