Vlookup and dependant data, please help

[execine]

Hello,

a few times people helped and it's really valuable, need your help further, would appreciate really much if somebody could help me out.

What i have is:
Different sheets which have equipments lists with name, s/n and value. I want to make
semi automatic invoice, so that from the drop down menu i select category of the
equipment, then accordingly i get lists from those sheets again in the drop down menu, then i select specific model and s/n , price fills in automatically.

Attaching the file for easier understanding,

thank you very much!
Tomas