I think this falls into the "Kids, don't try this at home" category. You expose yourself to noxious chemicals, and expose your neighbors to lead dust. Old paint is also a fire hazard if it's not properly disposed of. This is a job for the professionals.

My boyfriend has used a heat gun for small jobs on his UWS pre-war apt (Doors, window frames) and it works great. But I don't know how you would do entire rooms.

perhaps you would consider skim coating the walls, replacing the baseboards and only stripping the windows and door frames? you can hire the tools for the baseboards at home depot and get it done in a weekend. it is also pretty easy to do the skim coat yourself, you will just be sanding more bumps than the professionals have to. if your building was built before 1960 you can be just about certain that there is lead paint in there, but there are some strippers that can safely remove lead. never ever use a heat gun though, the fumes can be lethal.

here is a lead paint removal how to:
http://www.ronhazelton.com/howto/lead_paint_removal.htm

and a product that will strip lead paint:
http://www.paintremoval.com/paintremover.htm

i skim coated and stripped paint like this in my apartment, and my personal safety measures were:
remove everything from the room being worked on. put down a layer of resin paper, followed by a layer of plastic, followed by a layer of damp newspaper (it attracts dust)
keep a water bottle and spray the paper periodically.
seal off all doorways with two layers of plastic.
wear paper overalls when working in the room, and shower immediately after leaving it.
wear a respirator rated for lead.
clean up carefully and double bag all garbage.
discard any tool that was used in the paint removal.

great comment LauraWaHi.... very helpful; especially the tip about the newspapers

As I understand it, the safest thing to do with lead paint is to contain it rather than disturb it. I would go the skim coat idea because you can get into some serious issues (and fines I believe) if you go messing with lead paint by yourself. Contain it or call in the pros.

How much would it cost to let a pro do it for 500 sqf with 2 rooms?

I stripped the paint in my bathroom (house built in 1954, no children living in or visiting the house) using Peel Away 7. Having done this very small area (and using the 2 bucket, 2 sponge clean up method several times), I would say DON'T DO IT. It is so much work and in the end all of the paint didn't come off due to patching and a lot of other random issues. The product worked well but it was definitely not worth it. I may still have to skim coat the walls.

Just Don't Do It.

Aonther thing to consider is if you have bad plaster that needs to be repaired, skim coating won't repair that so you'll still have to disturb the lead paint to fix it.

If you use the correct method (something that doesn't create dust), removing paint, including lead paint, is perfectly safe and a reasonable DIY. My favorite methods are:

Silent Paint Remover:
http://www.silentpaintremover.com/
(lower temp than heat gun, so no danger of fire...I also find it easier)

I also like Peel Away:
http://www.peelaway.com/
(just make sure you get the correct formula for the material you are stripping...i.e. don't use formula 1 on softwood if you want to stain [instead of paint]...all of this is explained on the labels).

I used paint stripper (I don't remember the brand, but it had an orange color/smell and was supposed to be environmentally friendly) to do a door and frame in a 1920's apartment a few years ago. it was SUCH a pain in the ass. It took forever, was really messy, and stunk. Next time I would definitely hire someone.