Come on over to the bathrooms forum for some better advice. There are a lot of us there with old homes. I would use marine-grade poly/varnish to protect against water. If your window is actually in the tub you might want to consider switching to a vinyl fixed pane that is indended for showers.

Your tub shouldn't leak (!!!) but you would want to make sure the surface underneath is solid and doesn't cause a lot of deflection with the tub (and obviously not a style that needs to be set in mud).

If you're having a shower up there, you'd want to prioritize waterproofing the shower walls over anything else (since weight is a concern). Generally people think of tile, but it's important to use some kind of waterproofing under the tile (eg Kerdi, redguard) and then tile on top. Some people use solid surface (corian) panels.

For the walls, beadboard is always a nice historic touch. You could either do a stain grade or paint grade.

You need to be sure that the framing will support the weight of a tub that is full of water. If you still have original framing in the current room, you should be ok.

I would not get an acrylic unit if I wanted cast iron and tile. Proper backerboard and seals will keep water from going where it shouldn't.

agree fully with marine grade poly.

have fun planning!

I would absolutely check the joists under the tub and toilet areas to make sure that they can take the weight of a tub full of water. We refinished our wood floors in the bath room and applied marine spar varnish and water just beads up.

Weight of tub, water, and person/people.

Also, include allowance for Archimedes as an observer.

Before any of this we had a structural engineer come out and look everything over! (since our boiler is decades old and a "Hercules" we've named the house Hercules because it's 97 years old and still very strong, a great foundation!) We're having the engineer come back for the best placement of fixtures as well. I don't want an acrylic surround, but we were advised to avoid an iron tub because of the weight. I'm thinking an alcove tub and tile around it. We don't quite know yet what we're dealing with since this room was always a bedroom and the walls are the original fiberboard (no plaster upstairs!). I've been ignoring the actually planning for a bit and been spending my time on my true vintage love: lighting!