It all depends on what sort of ratio you need and space as to if its worth buying or making gearboxes. I tend to build all mine (access to a mill is ideal). Speed 900's all tend to use custom gearboxes as the ratios are only small - and therefore can get away with a single stage gearing. These can be really simple and easy to build like the ones in Satanix and beauty. It gets tricky when you need to go more towards a 2 or 3 strange gearbox and you need better equipment, alignment and the gears generally aren't cheap. It can save you a huge amount of weight though and get you motor positions mucho better.