Personally I prefer longer macro lenses on my D300 (which is an APS-C sensor camera). While I have a 70mm macro I rarely use it. For individual flowers and plants I like the smooth creamy backgrounds that can be obtained using lenses in the range from 150mm and up (I often use a 300mm f/2.8 pared with the 1.7 TC for larger plants). For wider garden shots I'm more often than not at 10-50 mm.
However we are all different and I'm more than confident that with your experience you already know which focal lengths you prefer, Arturo, as you have been using a camera for a long time. It's possible that for some macro images you may actually end up liking your 105mm effectively function as a longer lens, but if not, it's no big deal. You can have a cheap entry level APS-C sensor camera
AND a 60 mm macro lens (or depending on what you already have, perhaps a modern quality zoom lens would be better) for much less than what a full frame camera will cost you.
I have never encountered anyone interested in photographing small birds complaining about having a too long lens, so for this I think you will love an APS-C sensor camera. Worst case scenario, you could always move your hide back a bit
All of the current Nikon cameras will give you superb image quality, so you don't have to worry about that if buying a new one. Many of the older ones, no longer in production will also be great. It's more about finding out how the camera handles as the entry level cameras lack some direct buttons and dials of the more expensive ones, forcing you to use menus more and also of course the more expensive ones will have more functionality with your older lenses. The potential lack of mirror lock up on some models may or may not be an issue for you, but there are some partial workarounds for that. However you may be able to live with that for the cost savings
Only you can decide.
Have you looked into if your lenses are the AI versions or not yet? I assume they are, but best be sure.
It's a long time since I upgraded from the "ancient" D40x to a D300 and the reason for this was simply that I wanted mirror lock up for my low light tripod shots, often at high magnifications, sometimes using close up lenses and tubes. Such a silly thing to need to buy a new camera for! However looking at the compatibility charts from the links I posted above it looks like even this old entry level camera would work on manual with old AI Lenses. I'd get no metering as in more expensive cameras and the sensor is a bit noisy for longer exposures and at high ISO, but it would work.
Best would probably be if you could visit a store and ask if you could try a camera with one of your old lenses. Just so you could see how it handles. As money is a concern I'd try the cheapest one they have first, which should be the Nikon D3300. If you like it you could still buy it online or wherever the price is best
