You could probably check what pressure the wastegate is opening at by hooking up a compressor to it, but you should make sure you have a gauge on the compressor that is accurate so you don't end up damaging the wastegate. Also if the exhaust housing is cracked severely around the wastegate flapper area allowing air to escape when the flapper is closed you will have trouble building boost.
I wouldn't recommend that you "boost launch" your car while the car has the issues it has. Especially if you haven't even bought it, might upset the owner if something else breaks and you decided not to buy it. The only ways that I know of to build boost off the line would be a 2 step launch control setup or an antilag setup, both requiring you to mod your ECU.