Hi Danny,

I think you'll find the cost of belting will make using looped belts of this length un-economical. You probably won't find made up looped belts of this length unless large and expensive widths so will need continous belt with connecting plates. Last price I got for a good quality belt was £12mtr, there are cheaper but when it comes to belts you don't want cheap-O otherwise your in for a world of pain.!!
If plan on driving direct from pulleys off the stepper their will be a couple of issues.

1st Resolution will be low so some reduction will be needed, thou not massive if your just plotting.

2Nd The belt tension required over this length will put very high lateral loads on the stepper causing premature wear. Better if you drive direct from shafts with bearings at either end then drive the shaft with small belts under low tension to the steppers, this will also make it easier to create the reduction.

I would give Rack & pinion a look, I think you'll find it works out just as cheap with less hassle and much stronger if you ever want to upgrade and use it as cutting router.