If I were to buy a T5 conversion but the box has a few k's on it how much am I looking at to get it up to spec? Can I just get it serviced or something or do I need a full rebuild or reco? How much am I looking at exactly?

how many kms is a few?

Most people never changed the oil in the boxes. Fords service schedule says to do it at 80,000km. That is way too long, every 20,000km would have been better.
Buyer beware.

250,000+

The only way to know how the box is would to be either actually drive the car it's in on a road test, or do a full strip down, in the latter case you would just rebuild the box if you have gone that far.

I've got a low km 5spd, but i'm going to get the car it's in drivable to see how the box operates before i remove it. If it's fine, i'll just fit it to the EB & with new oil. If it doesn't drive well, i'll do a rebuild on the box.

Cheers

ToranaGuy