I've only recently been learning about wheels, so the bulk of my knowledge is based on reading and this forum. I've only built a couple of wheels, so my advice is based on only a little bit of experience.

ERD is important though, and you'd only be able to deviate by a small bit from it if you're going to use the same spokes. A 4 mm ERD deviation should equate to 2mm spoke length deviation, since you're splitting the diameter between the two halves.

You'd have to ask others here on exactly how much deviation from "perfect" that you can get before you run into build problems, but if you buy a rim that's got too low an ERD, you'll bottom out your spokes before you can get the wheel up to tension. If you go with an ERD that's too high, you won't have enough threads engaged. If you're pretty confident of the rim you need, then go with that rim. But if you're not sure, that's a $50 gamble.

Hopefully others will chime in on this one.

-Jeremy