Mine had several layers of paint as well. Even some of that tar crap on the tub. I had mine blasted certain parts at a time. Just went with what was ready to blast and what funds I had. I do recommend that you be ready for paint right after you sand blast it.
The axle I ordered from abctrailerparts.com. The prices are great and have what you need. Not too sure about electric and hand brake combo. I ordered mine to set up for electric brakes but doubt I'll go that route. It's a 3500# axle set up with 6 on 5.5 to match the tacoma wheel pattern. Now I just carry one spare at any given time. Having the hand brake would be nice.
I don't remember the floor thickness on the tub. Might be at the most 1/8". If you do replace the floor make sure the tub is square! You might even use something less thick and lay over and tack weld to save you $$ and time. You wouldn't have to cover the whole floor just most or areas of concern.
As far as the drain plug goes I can't remember where I got that from. It could have been Lowes
or Home Depot
? You could alway use a 2-3" piece of pipe that is threaded on both ends. Weld one end into the floor and cap the the other end. Should be easy enough.