I'd suggest these questions for a start (and as another poster has said,
find out what the club and its members is looking for, but these are
some of the important issues). My club has a K21, a Puchacz and a K13,
so I've added some comments about the K21/Puchacz to illustrate each
question.

1. Suitability for first training flights? (K21 is very encouraging
because it's easy to fly, Puchacz demands more accurate flying).

2. Suitability for later stages of pre-solo training? (K21 not so good
because it masks faults, Puchacz excellent)

3. Do we want to offer full spin recovery training? (K21 no good without
the spin kit [no idea how good with the kit], Puchacz excellent as it
does everything by the book but loses lots of height in the spin
compared with a K13)

4. Do we want to offer XC training? (K21 pretty good as less effort for
trainee to fly so can concentrate on XC skills, Puchacz OK but doesn't
penetrate so well into wind)

5. How easy is the ground handling? (both K21 and Puchacz are easy
because they have nose wheels and main wheel near the C of G; Puchacz is
keener to fly on its own in high winds so needs more care in gusty
conditions)

6. Cockpit size/weight limits/comfort? (Puchacz fits a wider range than
K21 in terms of height and breadth, weight limits are much the same)

7. Lookout visibility? (K21 is a little better from the back seat than
Puchacz, but both are OK)

8. Build quality? (K21 is excellent, Puchacz airframe seems solid but
pushrods, wheels, cables etc. are lower quality and our experience is
that it requires more watching and minor repair/replacement of these parts)

9. Handling/teaching vices? (K21 allows sloppy flying without real
penalty, Puchacz loses height rapidly in spins)

10. Ease of rigging/derigging? (Linked to the XC question, both are
pretty easy compared to a K13 if you can't find the special tool for
removing the main pins)

11. Will it fit in our hangar? (not a problem for a K21/Puchacz if
replacing a K13, but a DG1000 will require rather more space!)

12. Winch launch characteristics? (K21 is excellent, Puchacz has a very
narrow window of safe winch launching speeds)

13. Availability of spares? (Good for K21, reasonable for Puchacz [in
Europe at least])

I'm sure there are more questions, but these are the ones I recall being
discussed last time my club was considering what to buy.