There are a large number of "TNS"s on the Chipmunk - Technical News Sheets, detailing mandatory and advisory modifications and checks. Have a really good look at the log books an make sure they are all complied with (talk to someone in the group who knows). Some will be expensive if they haven't been done yet. There are quite a few NDA tests which need to be carried out on time cycles - e.g. x-raying u/c legs and tailplane attachments. These are ongoing, so you will be seeing the cost of these in maintenance bills sooner or later. I would also find out if their maintenance organisation looks after other Chipmunks - it's best that the guy looking after it for you knows them inside-out. Look at engine hours remaining, and make sure provision is being made for the overhaul. It's expensive and will mean the aircraft is out of use for a few weeks (or ask if the group has a spare engine to minimise down-time). In general, the engines 'use' quite a lot of oil (most of it ends up inside the cowl, and underneath the fuselage. If the engine bay and underneath are clean before you go for your test flight, it's an indication that the other group members take care of the machine. Try to establish how other group members treat the aeroplane watch them do the walk-round, and post-flight checks. Make sure you understand the airframe and fatigue hours. Major bits of the aeroplane have fatigue life limits at 30,000+ hrs. Seems a lot, but if the RAF put 12,000 hours on it, that gets a 2.5 multiple for fatigue hours, so you're now fast approaching some expensive maintenance. Then again, quite a few have had replacement wings, etc during their lives, so the remaining fatigue lifes on bits of the airframe may be all over the place. Make sure the cost structure of the group is making contingencies for unexpected maintenance bills - it will happen (and I'm still hurting from my own recent personal experience).
If that all sounds a bit overwhelming and disheartening, don't worry. By the time you've strapped yourself in, all that stuff is put to one side, as you head for the blue and spend a happy hour or so in such the wonderful aeroplane that the Chipmunk truly is. (...I'll be flying mine tomorrow).
Hope this helps.
ACL