Engine, tyres, suspension, brakes...the lot! Here's my advice (although there are others who wouldn't agree ;)
If you intend to keep the car for a number of years (and really, to be fair to subsequent owners :) ) run it in. Avoid high revs for long periods, warm the engine (oil) on every trip, vary revs, be gentle until the engine is fully warmed up...etc.
It's up to you.