The XL2 is probably the best bet for a reasonably priced DV camera these days, since it's a true 16:9 camera rather than 4:3 like the PD170: however for documentaries you might want to consider whether you want a more 'stealthy' camera like the PD170 rather than something big and blatant like the XL2. You should look at the Sony FX1 as well... it's high-definition rather than DV, but you can shoot HD, output DV from the camera to your editing system for the moment, and still have HD master tapes you can go back to if you need an HD version of your movie in the future.

That said, you might also want to look at the used camera market: while it's always perilous, a lot of people seem to be dumping PD170s and the like for FX1s at the moment so prices should be pretty good.