have you checked The Ukulele Site(aka HMS) they have the largest inventory of Ponos...and all you have to do is ask for a LowG to be put on before they ship it
With that kind of budget, you can get a new one as long as you don't want a slot head or abalone rosette... See what people have on here, but you aren't limited to second hand. Don't stress about a low g string. You can pick one up for $5 to $10 and they're really easy to change.
By the way, I'm not saying anything against Ponos because they have a fantastic reputation, but is there a reason why you are set on a pono? I recently went shopping with a similar budget and found that there are quite a few good options around that price range or even below.
I've got a Pono MT (satin finish) purchased from HMS/theukulelesite.com) last Spring currently strung with Southcoast LML-NWs. You can buy a new one for $351 plus shipping. I'd be happy to entertain offers for mine (I've recently discovered baritones and need to thin my collection). A year on, my Pono has developed a nice patina (the new ones are very light and rather plain looking) and the wood has opened up noticeably. Still, Pono's aren't the loudest/brightest ukes around, but they very well built and put out a nicely balanced sound.
tkt