swarfboy,
your question is a bit of a "how bigs a hole" one. but I would say that you would be looking at 120 watt motors
running at 2-4000 rpm geared to give you a maximum of 3-4000 mm per minute a 250 cpr encoder would be about right
with a g320. and thats with a good pentium running up to 45khtz.
gecko's wont run brushless motors,you are looking at mega bucks for a brushless system.
remember you can fix encoders on to practicaly any dc pm motor to make a servo as long as they have a minimum of 12
poles they will run ok without cogging, I have modified a lot of "standard" motors successfully so I know it works.
In short you can build a servo system for really not much more than steppers.
I think you will struggle to build a brushless system without digging very deep into your pockets!