Backlash adjustment for routers is more or less useless, it's more for laser engravers. You will have to physically remove the slop in the system or cut a lot slower producing inferior parts until you do.
See if you can adjust the pinion to mesh in with the teeth of the rack better or replace the pinions. Some types of ballscrew nuts can be tightened up or the the load nut at the end of the ballscrew can be adjusted up. Once you get all the slop out, it will feel like night and day cutting with an accurate machine. It's well worth the time, there's not much expense.
Where expense comes into it is vacuum pumps. It all depends on what you want to cut and what you can afford. Do your research, do lots of reading on the net and set up your system with the idea in mind that you will be adding in more vacuum down the line. You can never have enough vacuum. I started with one reg blower on the 8x4 table and ended up with two reg blowers and a rotary vane...it's pretty good, but you always want for more. Look for good second hand stuff, there's lots around.
Two smaller pumps are usually more versatile and economical than one big one as you can zone your table and use what you need at the time. Our biesse has 3x 300 m3/h rotary vanes on a 4.3mx2.2m table with 8 zones...it's very versatile as you can zone it so all that vacuum is concentrated into a 6 x 1 foot area if need be to hold down brass sheet etc, if need be.
Head on over to cnczone or the shopbot forum for plenty more info.
