In this price range you run a very high risk of buying a dangerous heap of seriously corroded junk. Eunos roadsters were not rustproofed. All MX-5s can rust quite badly and the sort of car that your budget buys will be at least 20 years old. So the body is the most important thing. Engines, gearboxes, axles: everything else can be fixed and if the timing belt has not been replaced in the past 40,000 miles, it needs to be.