heya mate
1) if it is a turbo, find out,
who installed it
papers on the fiting
any modifications to the engine itself
if its a w reg then itll be an m44 engine
turbo lag is from the size of the turbo (larger) needing time to spin up at lower tourque / revs,.. its normal but not good .. some would say it shouldnt even have lag on a small engine. it just means the set up isnt ballanced to the car..
the rack may be an issue. if its a new rack then it may cost you dollar if you cant source a used from a breaker. this would be the only sensible option imo.
The air con may be.
duff klima kondenser (ac rad)
the thermo resistor
the compressor.
the belt
the gas (but if its had a refil then rule this out)
it will need a dose of contrast gas (mixes with the freon and shows illuminous leaks) to see if theres a breach
do you have a sales aqd or link to the car?