I have owned three Comanches over the years and currently have a PA30 Twinky. They are good solid airplanes. The 250 series (I had a 1959) were not near the aircraft the newer 260's (after 1968) were but they are still very good fast aircraft for the money. The PA30's are really great if you want a twin. I burn about 14 gallons / hr (and as low as 12/hr) and still have a good 160kt cruise out of ours. If you add some of the speed mods from LoPresti http://www.speedmods.com/index2.htm or Knots2U http://knots2u.com/ they really become very fast and very economical.

Due to the age you may well find that the avionics in some are old as well and many have not had the panels updated. Find one that has good avionics and a panel makeover you will come out with a much nicer aircraft.

The really only caution is to be SURE the gear has been maintained and adjusted properly. This is one area that the Comanches are weak in from my experience. There are correct and proper procedures for maintenance and many Comanches just haven't had those done properly. A mechanic who knows the series is required to get these correct. Some of the 250/260's may also need attention on the stabilator torque tube horns there are some issues with them see

http://forums.piperowner.org/read/1/103249.

There are several AD's and you do want to be sure ALL of them have been done! Be careful because there are some "Pencil" fixes on some aircraft you want to assure compliance.
Overall for speed, economy, room and hauling ability they are a tough aircraft to beat! (OK so I am biased.) Check the specs particularly with some of the speed mods you will like what you see. If you get an early one with the 60 gallon tanks, add the tip tanks. With 90 gallons you can travel a good distance and still have fuel. My Twinky has 120 gallons available and can go 9 hours with no real issues. (I can't but it can.) Oh, the newer 260's will have 90 gallons without tips! Do expect 2-3K for annuals and don't be too shocked at a few 6K ones. You may come out lower some years but the aircraft are getting older and good maintenance is worth it for the performance/cost on the PA24 and PA30 series.

PS IF you just want a really good, low fuel burn, better than Arrow performance the 180's really aren't too bad either but they certainly aren't the same as the 250/260's.
Parts really haven't been to much of an issue at least now. I do suggest joining the ICS (International Comanche Society) they have good help and resources for owners.

http://www.comancheflyer.com/NS/