They can take a beating. Really the only problems are broken ears/cracked bolt holes are what to look out for. Should be recognizable from a picture. Really they just sit on top of the motor. There isn't any mechanical/moving parts to wear out etc. And yes a mechanic could do the gasket match and isn't really necessary. It just clears up any air turbulence entering the intake and for a street car(or even a race car for that matter) it wouldn't be noticed. Probably 5-10hp at peak power at most.
Again before buying individual parts make sure to research everything. Also it will give you time to increase your savings. With a well thought out plan-parts acquisitions-you can have a very fun car. Typical results are anywhere from 250rwhp-350rwhp(gt40 iron heads vs good aluminum) and up to 11.xx 1/4 mile et's(expect 12.50-13.00's as avg)