well as far as the head gasket goes you could do a block check (little bottle filled with fluid, bung it in the radiator with car warm, if fluid changes colour you have exhaust gasses present which = bhg)
another really good thing to do would be a compression check on it... usually about the 150-180 mark depending on wear bla bla bla... make sure they're basically even... toyota manual suggests any more than 14psi difference between any cylinders requires a rebuild and anything lower than 128 requires a rebuild... but in my opinion 150 mark is a good one to shoot for
turbo... well all you can really do is check for shaft play and once the car is warmed up you could give it a rev then see if smoke comes out on deceleration.. this is generally when they blow smoke
boost... yep 5sp usually bout 6.8psi, auto bout 5.8-6.0ish
take it for a run see how it goes
they do have a stock boost gauge... generally accurate but slow to react from memory on analogue
also check the water bottle, are there any old water stains down it... usually a sign of previous overheating etc
also check for rust in the spare wheel well... they like to leak in the tailight areas
that's all i can think of contributing at 2am hehe
good luck on the supra
