Kyle,
- Rotor type is irrelevant for 99% of your potential audience. The only aircraft that I know of that have a different 'rotor' available are the Lama and S61. The 'High altitute' to which you refer is probably the HA tail rotor kit on the 206L4. Maybe a section covering kits would suffice.
-There are no twin-engine piston helicopters. I'm sure someone will come on here with some obscure home-built just to prove this incorrect, but there are none certified for use here.
- There are two 3-engine helicopters that I know of, the EH101 and the CH53E. I doubt you'll ever be selling either on your site. To date, the quad doesn't exist.
BTW, it's a HELICOPTER. 'Choppers' have two wheels, handlebars, and an a$$hole sitting on them.