I have limited knowledge on the finance side of things but have had to deal with the maint in a 3 aircraft school.
C150 under powered and cramped
C152 cramped
C172 Expensive for a ppl but has a bit more room
All the cessnas have good parts availablity and there is good knowledge base of engineers out there who can fix them. Costs are pretty low which is why you see so many of them. Gear is very good at absorbing student landings.
Robins good size and handling spares a bit more expensive. Engineering knowledge base reasonable. FI's need to protectthe geat though
Pipers. Again good engineering base uk wide spares can be had pretty quick.
PA28 roomy but to stable. Low engine sizes are powerfull enough for 2 up no probs Fuel selector in daft place though. Under carrage needs looking after by the FI
PA38. Crap trimmer, very cheap, bad reputation, handels very well, roomy, all the controls the FI can get to without major hassels. Gear is very good.
Katana. Like flying a hairdryer. Good viz, pain in the backside for getting old or less nimble students into. Its plastic so hard to fix if it gets bent. Problems with brakes in the past. Handels ok but has no spirit.
IN all i would go for the PA38 for cost ease of fixing and pax comfort. Has a bit of spirit about it. And can handel 7 hours a day 7 hours a week of teaching circuits with no problems.
MJ