Multimeter

It’s probably a good idea to follow the specifications as listed in the more advanced category here. But if you really only do want it for basic stuff the only real important thing (which I could never do without!) is the audible continuity tester!

For basic testing all you’ll need is to be able to measure +12v so any multimeter will do. I recommend buying a digital one though, just because they are as cheap as the analogue ones and are obviously easier to read quickly.