A3000s & A4000s are rare to find and attract higher prices. There are a couple of A4000's on amibay at the moment.
A2000s offers good expansion (more so than the A3000) and come up for sale more frequently and are more attractively priced. You can expand them quite easily too - Terrible Fire; PiStorm etc
A2000 might be a good option.
Weed
