there really is not much to it.. anything over a 4 stage is overkill. + hard to mount. they get pretty long.
also the pump style is important aswell.
a simple aviad spur gear pump is basic, not pull much vacume but will take alot of crap.
or a uber expencive auto-verdi roots style pump is better but higher maintenance and pull heaps of vacume.
anything over 12-15 inches of vac will ware out little ends
my fave is dailey engineering ones. robust and roots style scavenge.
you also want one with a built in scavenge manifold for easier plumbing.
bare in mind the price of hose and fittings. eg -12's and -16's range from $50-$100 per fitting for anything other than a straight.
hose -12 $40-150 a meter and -16 $50-230 a meter
our race car at work used 9 meters of -16 2m or -12 and 5m of -10. 10 -16 fittings 4 -10's and 7 -12's...... or and 3 -16 45 degree bulkheads and a -10 bulkhead.
the pump is a SMALL PERCENTAGE of the cost.