The 128 will probably work just fine today. They don't have the notoriously dangerous power supply of the C64. You might want to add a cooling fan to it, or some heat sinks on the rarer chips. To be safer, you could even test the voltages on the power supply before plugging it into the machine, and open it up to look for leaking capacitors, but it's not really a machine that fails much (yet).
The SE/30 needs to have its PRAM battery removed, it is a bunch of lithium-acid waiting to leak all over the logic board. The electrolytic capacitors on it are also fairly notorious for leaking and destroying electrical traces, but there's not much you can do about that without learning to use solder.