I was standing at the wrong place at the wrong time and had "idiot" stamped on my forehead.

I now have two old, metal framed, oak benches in my yard I need to refurbish because I promised my daughter I would give them to her after I did so. Now that I'm doing the project, I realize, the wood is too far gone for any purpose other than kindling. I am almost positive it would be cheaper to buy new than replace the boards. I have yet to by the boards because the price at Home Depot was $65 a plank for oak and I need seven of them. Is there a cheaper, yet still durable wood?

The benches are missing a few metal cap bolts, and one of the angled braces, I assume I can get those at any hardware store.
I sprayed the nuts with Tri-flo yesterday, so hopefully, they'll be ready to release today. If not, is there a better lubricant?

The iron frame doesn't need repainting, but if I'm getting new boards, it'll look funny if I don't repaint. Her favorite color is purple, so I think I'll put down a base coat of white, then paint purple over it. Any suggestions on the type of paint?

I go back and forth on a nature finish on the wood, or slapping paint on it. Thoughts?

My friend uses WD40 and/or Liquid Wrench when he needs to release rusted-on bolts. I guess the iron frame is in good shape, but for rusted spots, the spray paint Rustoleum Rust Reformer will stop the rust, and then you can spray or hand paint over it. I'd go ahead and get some dimensional hardwood boards (preferably those that you can pick out yourself - check them out carefully to make sure they're straight and in relatively good shape). Cut them to length and, if necessary, sand them a bit. Red oak stands up well it's what they use on fire tower steps. Use a good quality outdoor paint.
It would be much easier to buy her a couple of cheap benches, but these will be made by you and be much more valuable to her, and to you. Just imagine how nice it will be and how many tales you'll have to tell her when the two of you are sitting on the benches together in her garden.

Her husband is a fireman, so red oak would be appropriate! Thanks!

If you have access to cypress, it weathers beautifully and is durable.