Unless I was tightly price restrained, I'd go for a Gen II, '04-09.
The following from my driving an '08 prius and being a
PRIUSchat.com member for four years:
But even then I'd look for a '06-09. The '04 & '05 have shown
a disturbing frequency of failures in the "combination meter"
the info screen, having to do with bad solder connections.
It's fixable but you have to take it out and send it off for
repairs. IIRC, dealers want ~$2K to repair.
There is also an unconfirmed possibility that after '06 the
traction control was made less sensitive allowing some
wheel spin in slippery conditions.
Of late, there have been a small but unnerving number of
Gen IIs from areas with long hot summers having HV battery
failures at maybe 80K miles or so. The failures most often are
single cell failures. The e-smart DIY types can replace single cells
at maybe $50 each from ebay. Reconditioned used complete HV
batteries go for ~$1K, but a dealer won't install one.
That said, there is every reason to believe that the HV battery
is good for at least 200K miles.
Unless you are very e-savy and well experienced rebuilder,
stay well clear of any salvage title Prii.
Lastly, there are some recommended service/parts replacement
that occur around 100K miles. These come to maybe $2K at a dealer.
What services you need and what you don't - PriusChat Forums
I'd recommend that if you're considering a used Prius you spend
some time in the
Gen II forums at PRIUSchat to get a feel for
things. Also use the search function to dig into the archives for
specific topics, ie, combination meter.