Toilets, Sinks, Showers, Tubs and Disposals - Replace a really bad shower

I'm going to dig into a shower in a basement bathroom. It's a small bath with an older fiberglass 3 piece shower that has had leaking problems for who knows how long. The dry wall is on two sides and from what I see, is really bad, water damaged. I noticed at my local do-it-yourself place that there are a couple different wall boards available. A couple say they are for high moisture areas and one (the $13.00 a sheet) claims to combat mold. What would be best? Also, how do I measure from the wall to the drain so I get the correct sized base? Thanks for your advice.

You can use green board dry wall. Mud the joints and seal it with clear seal or enamel paint.

Measure from both walls for the shower pan. Measure dead center in the drain hole.

You can also buy shower pans without drain holes and cut your own.

Duke of ....

Thanks Duke! I've got the shower and wall ripped down and there wasn't as much water damage as I feared. The base was terracotta or a light cement. There is a ring (I'm guessing was part of the base) that looks like lead. How do I get that off and do I need a vapor barrier between studs and the green board?
Thanks