Tubes deteriorate with use and age so it's not a good idea unless you use it in anger and need spares to buy a tube when you buy a machine.
In 2 - 3 years time when the tube goes your spare will be half ? used by age.

CNC Zone has some good post on these machines.

General Laser Engraving & Cutting Machine Discussion - CNCzone.com-The Largest Machinist Community on the net!

Look for Chinese 40w laser or Moshidraw which is the software they come with.