"A relay design would be pretty mush the sme for 110V or 240V."

Yes the principle is identical - just make sure that everything is suitably rated.

Don't forget that "240" is the nominal RMS - so the actual peak voltages involved are much higher.
Hence use at least 400V ratings for any semiconductors on the mains side.

"Use optoisolators (but of course)."

I don't think that's entirely necessary if you're using a relay to do the switching - the coil is inherently isolated from the contacts.

It's probably best not to mount the relay on the same PCB as your low-voltage electronics!

"Also does anyone know how long an RS232 cable can be?"

That depends on the cable, and how fast you want it.
The current RS232 spec does not state a length limit; rather, the maximum capacitance.
This was mentioned a while back - try a search.

Rather than RS232, have you considered the Dallas 1-Wire?