CNC gantry mounted laser questions

Hi all,

I'm interested in building a 4' X 4' Gantry type CNC laser. The
Motion control and design of the gantry are pretty much covered but
I'm wondering about the laser unit itself. I'm planning on building
one for cutting thin stainless and other metals.
What types are there?.


Which are the most economical?


Which are the most durable/reliable?


What are some good sources for possible surplus units?


What are the safety concerns?


Which require the least amount of maintenence?


Which are the most efficient?


Any experience building one? What should I watch out for?


Any information you can provide would be appreciated. Thanks in
advance.

Ron