I've got one. It's a great car. They do a Rallye version to but that doesnt have air con, electric windows etc, though it is a little lighter.
Generally 2nd hand ones will have under 100k on I would have thought. I brought my Phase3 99 plate last year with 55k on the clock for £3000. I did get it slightly cheaper than usual though as I brought it off my brother and have always sold him bikes cheap in the past. I would still say you should be able to pick up a decent one with under 100k on it for £2500ish maybe. All depends on what phase you want. You will find that a lot of the older phase models have had bits changed so they look like a later phase model anyway by now.
As for reliability. The cambelt as mentioned should be changed every 30k regardless of what Peugeot say. Its a car that is hard not to put your foot down in every now and then as once you have done it once it just screams at you to do it more

Fuel economy wise, there is definately something up with the one above. On a run I can average 35-37mpg and if I drive sensibly about town I would say I'm between 25-30mpg still. When I say sensibly I mean not having a traffic light grand prix at every set of lights. Best I have gotten out of one tank is 440miles until the light comes on, though most seem to get about 350 on the 306 site. I usually hit around the 400mark though personally. One thing to look out for is where the fuel hoses join upto the plastic 90degree elbow joints under the bonnet on the RH side of the car as you sit in it. The crimps can come loose sometimes. When I was playing around i knocked a hose and fuel squirted out!! so I have put cable ties on these connections too now and the problem is solved.
One thing is I did notice my economy got slightly better when I fitted a K&N filter (induction kit one) but I have stuffed it into the original airbox to prevent heat soak, Blueflame exhaust system (cat back) with a de-cat too. That said it may be because when I fitted this I also fitted a new Lamda sensor into the de-cat and maybe the older one was on its way out.
The only other reliability thing I can gripe about is the electrics. It is only in winter though when it is cold I get any issues for some reason. Like a digit on my ODO doesnt show up until the temp in the car rises a little. Sometimes the central locking plays up too for some bizarre reason that I cannot figure out, though tempting fate here as of yet it has been fine this year so far.
Performance wise, well I love it. 167hp standard (dunno what mine will be now, maybe 172hp the mods i have added wont have added much power though it does pull harder now) and the handling is just amazing, especially with the passive steering effect. Like the 205 gti they suffer from lift off over steer but I find you do really have to provoke it to do this and as I drive quite smoothly have only ever done it once in 15000miles when i really tightened a turn around a roundabout once, though I didnt lift off I stayed neutral it still broke away. Its easy to recover though (well mine was) just accelerated and straightened the wheel a small amount only and it came back in line. It goes well, 0-60 in 7.2 and stops well though that is another thing I have changed and put Tarox sport disks and Tarox sport pads on the front and now the brakes are pure evil! Can never have to good a brakes though.
I'm not sure what you drive now but you may find eveything in a gti-6 feels quite heavy. The steering is quite heavy even though it has power steering but it also gives a lot of feel which is great. The clutch is also rather heavy too. Sometimes this is due to the clutch cable which is routed near the manifold and the auto tensioner mech dries up. Changing the clutch cable can do wonders, though on *** gti-6 is a pain of a job as the air-con tubes are right in the way so be prepared to skin knuckles if you ever change it yourself (which I have done)
It really is a nice car to drive. I love the mods I have done. The exhaust and filter give it an amazing engine note on hard acceleration but when driving normally you cant really tell that they are there (bar at 3000rpm when the exhaust has a natural growl from inside the car) It is kinda sleeperish too as it doesnt look any different than a 306 HDI so people dont expect a 306 to go like a gti-6 does

Just make sure you have decent tyres fitted so you can exploit the wonderful handling that this car has!