Need rotary encoder for printer build

A low cost 400PPR quadrature encoder would need to be on a 1/4" or smaller pulley, which is unlikely. You'll probably have to look at the more expensive 2500PPR encoders, which could use 40mm pitch diameter pulleys and get right around 20PPMM. A slightly smaller pulley, say 32-35mm, would guarantee you the accuracy. A 2000PPR encoder may be a little cheaper, and put you in a similar ballpark for pulley sizes.

But you'll have to calibrate the system regardless, because even if you set up everything to get precisely 20PPMM, it's almost impossible that it'll be 20.0000PPMM, and you'll get cumulative errors.

 

Anything industrial grade will be 24V tolerant, but getting it below the $200 mark can be tricky. Any quadrature output encoder should produce essentially the same results, though, so trolling eBay should yield something usable.

Depending on on your anticipated running speed and its resulting signal frequency, you're likely to be able to find a level shifter that can operate quickly enough to interface with 5V devices as well, which may be the cheaper option overall.