PA-30 are among the better light twins that you can own. They are reliable if were looked after (after all the are about 40 years old) with very few recurrent ADs. You burn ~ 14gl/h and fly ~ 160kts. If the aircraft has tip tanks you can fill them and the main (90gl.) and load the aircraft as you have 60gl. only as the fuel in the tips does not count towards the W&B (I forgot the explanation but it is to do with the loads on the spar).
Most of the parts are readily available but some like the trim tab are more difficult. There is a support group 'International Comanche Society' (
http://www.comancheflyer.com/) which can help you with advice.
In any case, before you buy make sure to pre check the aircraft by a mechanic that
KNOWS the aircraft, there are some good shops in the UK, if you buy ask. The undercarriage, the elevator tube and bearings and the high sheer rivets needs to be inspected properly. Also, some of the twins can suffer from corrosion near the exhaust area. If you are looking to buy an Australian PA30 make sure to open the panels in the wing just behind the exhausts. The Australian CAA forced the Australian owners to cut in this inspection panel, as a result gases found there way in and attacked the spar. It is a great aircraft but like all machines it has its corners and you have to be aware of them, some in common with most light twins some are unique. In any case train yourself with an instructor that knows the aircraft and not one that says "All twins are the same and I have flown many".