[Very limited experience here but I'll tell you what I know]
A friend of mine has a PSP that was at like 2.2 firmware or something; well I had bought the Lumines disc a few weeks ago for $15 used (even though I don't own a PSP yet) because I know how much Lumines goes for on fleabay now (like $40-$50 used). Lumines has an exploit (similar to the GTA: LCS one but better) that lets you downflash any firmware down to 1.5 to do..practically anything.
(Since the Lumines exploit was found, there is now a 3.01 firmware that apparently corrects the Lumines exploit, so now it'll flash up to 3.0 down to 1.5; I don't know if anyone's figured out a way to downflash from 3.01 yet but I'm sure it's only a matter of time.)
I'm sorta in the same predicament...I want one of the new ones but I'm sure it won't be very hackable (at first) so I may end up buying a used v1 PSP as well. I'm sure the v2 [slim] PSP will ship with 3.01 or newer firmware, but again I'm sure someone will figure out how to crack it eventually.
If you get a used one, as long as the firmware is >3.01, you're good to go. For my friend's, we got the 3.0 update off a PSP site, then used the Lumines exploit to go from 3.0 -> 1.5 (seems like the easiest way to do it). Of course now that 3.01 is out, if you use the update feature on the PSP itself, it will download 3.01, but you can find official 3.0 firmware out there to save onto memory stick and load to update it offline.