What size 120: 6x4.5, 6x6, 6x7, 6x8, 6x9 cm... ?

Durst M600's are probably the nicest small, easily stowable MF enlargers. They made series for 6x6 and 6x7, not so sure about 6x9.

Look for what's available on ebay. If you get a Durst make sure it comes with negative carriers and lens boards - advice for all enlargers, really.

I would stay away from the Printmakers - they are a poor value compared to what else is available in used enlargers.