subnoob

I’m considering a purchase of a 2002 Impreza TS 2.5L, it has 110K miles on it, and it seems to be well maintained. The seller has priced it $20 less than the KBB value of a “good” condition. However, the seller has not changed the timing belt, and it has struts that need to be replaced, which I took into consideration when bargaining down the price $700 lower than the KBB value. However, I am now concerned about the gasket problem, which from what I understand is an expensive repair. Is the gasket issue something that is a scheduled maintenance that I could have expected to be taken care of in order for the car to be in “good” condition? I do not want to insult the seller by asking him to renegotiate the price if that is something that I shouldn’t expect from him. To buy/bargain or to let this car go? I'd appreciate any input you have on this.