I am a firm believer in one machine for one process. So I would suggest you buy a cheap chinese laser cutter even if it is at 40W. I went with the 50W which was slightly dearer but had on board controls.

In terms of what you would need to convert a plasma to laser:

Powersupply £100
laser tube ~£300-500
optics £100
holders for the optics ~£100
Waterpump or chiller for the tube £10-500
fume extraction method £20-1000