A fiberglass tub is easy to remove if you're demo-ing the room. They're lightweight and easy to cut. A DIYer could do a bathroom job, especially if you did a lot of research and asked questions.
As far as waterproofing, I'm a fan of the Kerdi mat by Schluter System. It's the only system (that I've found) that's rated for steam, which means a plain old shower isn't going penetrate, cause mold, etc. And, tile can be set on it right after it's applied.
With a roll on, like RedGard, which I used to use on jobs - years back- it always required two coats. That was time a lot of people didn't want to spend, so they'd roll one and do finish work. Little did they know that there was little, if any, protection beyond the waterproofing substrate. I've seen this quite a bit on remodels.
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