Please post pictures

Pics for sure . I happen to love arcrylic showers or tub/ shower combos if budget is a big issue . I do need to know the size of the spaces for sure. In a 35 yr old shower there could be some surprises behind those walls so gut the whole space IMO and then do what you like .If you want help with anything you need to post pics

See if you can use standard off the shelf vanities instead of something custom and put the money you save there toward complete retiling. It probably is best to rip it all out if you're going in. We found we had a very small leak in a shower floor in a small bathroom (dating from 30-40 years previous) that we didn't know about, but it was dripping water into the ceiling below very slowly over time which is never a good thing.

I have to say I wouldn't want an acrylic shower/tub combo in my bathroom, but there is one in our secondary bathroom where we live now and it's really not bad. It's a Kohler. I think it's been there 25 years and it still looks new. So that really might be a good, inexpensive option for a guest bath as Patricia says. They're easy to clean.

Even with pictures, ROI is best determined by a consult with your real estate agent. We flip houses as part of our practice and this answer really does depend on too many variables to judge just from a picture alone.