which mirrors?

No idea....
I've a 50w redsail m500. Need new mirrors but no idea which ones (I'm looking on ebay).
Any help would be appreciated.

The mirros from your other machine should work, they're usually the same size. If they are no good search for K9 CO2 mirrors on ebay.

Does focal range not matter? There seem to be a range of options including wattage....should I not get mirrors for 50w?

The 40w mirrors will do the 50w(up to 80w I think) too. The mirrors dont have a focal fange, just the lens.