Having never flown a Robin before I have no idea. Firstly I would defenatly ask the opinion of those who are going to spend money with you. If they arnt sure arrange to bring aircrafts in for viewing, and invite your members to take a look.
The Diamond Star (TDI) is a very nice aircraft and i hope to have a go at one soonish. From what ive heard from a light aircraft leasing company they think that a DA40 TDI could justify the origanal cost in the reduction in fuel costs (presuming you can get D fuel). In the UK the DA40 has had various engine problems.
The Cessna 152 / Piper Warrior combenation seems to work (especially at sunny shoreham). As long as you insist one is for training (C152) and one is for touring (PA28). This way new customers see that you are trying not to rip them off. The 152 is a very good aircraft to train on although with the market at the moment you will probably be looking to put a new engine in one you buy.
Hope This Helps!