I recently picked up a set of very good/clean rider Black '83 MX1000's with Black '83 Tech 3 levers and Black DC '83 dated cables for 100gbp posted.
MX900's are great if you ride slammed and don't have the flex that 1000's give. It all depends when building the bike, what size you will need. Chainring size and freewheel size all play a part on where your wheel will sit in the drops thus, deciding for you what caliper you will need.
As said already, 1000's give you more flexibility overall when building a bike, making them more desirable
That said, mint black setups are hard to get hold of so if someone needs black, best bet is you will sell the 900's no problem
Tj