What I can tell you is that you would most likely want a fiberglass trawler or the like.

The tub had a fiberglass liner over the old tub and the new one had cracked. 

The tubs and showers are cheap plastic or fiberglass- I think they need to be replaced, along with all of the bathroom fixtures- I'm hoping to keep the vanities, which match the kitchen but put a new top on them.

I just did a quick glance and the fiberglass tub on Home Depot was $150.Account Closed Thank you and I agree.

You need to insulate within that cavity with unfaced fiberglass insulation, and then when you put in the window, you will generally want to use Great Stuff window and door expanding foam along the sides as well.