Soooo...i am taking a shower last night before work, and I take a step back and feel the tub/shower unit "give". When I finished I lift up the bath mat and voila! There's a crack in the stupid thing!

This is a 40 yr. old tub/shower fiberglass combo unit that has already been patched. It's time to replace it! So I go online and find a lovely one piece shower only unit that will fit the space perfectly. It has a bench built in, a grab bar, shelves for stuff and it can be a "normal" shower fixture or fancy multi headed shower fixture ( you cut the holes depending on what you want)

My question is: What is under there?? I know regular shower units usually have a shower pad under them. Would a tub/shower unit have a long pad of the same stuff under it? Is it possible it sits on the studs? If it is a pad, does it have a drain separate from the actual drain that is hooked to the unit? It's over a crawl space, but I AIN'T going in! I'm just worried about being able to replace it myself.

The bathroom needs a lot of work and I figure if I have to replace the tub/shower unit, I may as well do the rest of it too. I am not looking for fancy. I want safe and sound. I am NEVER selling this house, so I want to replace the tub/shower unit with a walk in shower. I figure that ALL the wallboard is regular gypsum wallboard and NOT cement wallboard or greenboard, since whoever built it cut corners everywhere else! I will have to replace that all too. I may as well insulate better since the walls will be uncovered. I don't have a fan in there so I may as well replace the light fixtures and put in a fan. Can I run the vent from that to the stinkpipe or will that cause a problem? (I have a metal roof and I don't want to cut it!) Would I be better off with a wall unit fan that vents through the wall with no light attached? Can I run the power for that from an existing light so when you turn on the light, the fan comes on?

Flooring: I live within 20 feet of the lake. Mud/dirt/grass/flooding/etc.
I figure sheet vinyl flooring is the best bet, but does anyone have any experience with engineered hardwood? Is there a brand that holds up well under moist/wet conditions?

AARGH! I wasn't planning on doing the bathroom right now, and I cannot nor do I want a contractor to do the job cause I don't trust them! NOw I have to do a crash course!! Any suggestions/answers are truly appreciated!