I saw your last post but was too busy to answer besides the fact that I think you are way in over your head and doing this wrong. You will be wasting your roof before this is all done and cause more trouble than it is worth not to frame it all up now. How are you going to enter the basement? I suppose you think you can jack up and entire roof to start putting floors under it, I'm getting a laugh out of that.
That said, you need a contractor or architect to figure out where your load bearing walls are going to go in the basement so you can pour the footings for them before the basement floor(if they aren't supported by footings, then your floor will break up under the load, your upper floors will sag, and that sagging will mess up your floors, walls, and ceilings). To do this it will help to have the floor plan for the upper 2 floors so the load bearing walls will all tie in for proper support. Then decide what size floor joists/trusses you are going to use to span the basement and use the load bearing walls to support it. (I assume by main beams, you are referring to the floor joists and supporting load bearing walls) If you insist on doing this, install your floor joists/trussed, then your subfloor, then your roof trusses.
Put 6 mil plastic sheeting under the basement floor concrete. Wouldn't hurt to lay some 2" insulation board under the floor. Don't forget basements have to have ingress/egress type windows for all basement bedrooms or they won't let you live there. You ask about sealer and vapor barrier, if moisture is a problem fix it from the outside of the basement wall using a good concrete sealer. Insulation on the basement walls wouldn't hurt if you are in a cold climate or wrap it in plastic sheeting after putting on your concrete sealer. Around here they usually use a black tar to seal basements with. Also don't forget to put in the drain tile around the basement with a sump pit.
Drylock sealer on the inside won't hurt (and not necessary if no moisture comes in) but moisture infiltration starts from the outside. If you live with a high water table, then do everything you can now. Vapor barrier goes on the inside of the insulation and under the drywall (not on the concrete wall).
Saw a website or two in floors over concrete but you basicall need 2by floor shims with a plywood subfloor.
Good luck and get someone to look at your plans. Contact the lumber yard where you have your roof trusses built and they can lay everything out for you on sizes, etc. They can tell you what you need for floor joists/trusses and how much they span depending upon your load bearing walls in the basement. Good luck again as I keep chuckling thinking about this project.