Does the following look like like I have my bases covered? Should I paint the wallboard behind the shower enclosure? Should I put some other sort of membrane between the wallboard and shower enclosure?

House is 60yo and it looks like original wall was a cement covered by tile (ugly pink - glad to see it go . A repair was made over 35 years ago where they built up the studs to mate 1/2" white wallboard to existing 3/4" wall two feet up around tub.

My plan is to rip out all of wall and replace with green board and put a plastic (we're poor) shower enclosure over top of the wall board. The tub mates against the studs with an upward curl. The wall board will mate against that top of curl, then I will fill in that curl completely with silicon based sealant which will seal bottom of wall board.