All radio systems are pretty tolerant having an average ohmage from 2ohm to 8 ohm. You would have to series or parallel several speakers to exceed these ohmages. Realistically it's the current which burns out the preamp on the head unit, rather than the ohmage load the head unit is seeing, its simple ohm's law, voltage, inductance, resistance.
For a basic install, a multi meter would be nice to have, but a test light is all you need. If you were linking a headunit with a pre amp equalizer and individually sending freq's to each cone then you would need to invest in knowledge in knowing what your creating, and an ohm meter.
Running a new constant ve wire, a switched ve wire and an earth is all you need to do. Then run new wires for the speakers. If the ve wires are not up to scratch, you would have to assume that being routed in the doors etc, the speaker wires would be the same or at least worse. So running new speaker wires would be a good option too.
For a hint, the head unit usually has the wiring diagram on the unit itself, or the documentation or the box at least. Getting in and having a go is the best option, though damage can occur if it doesn't go right.