Hi Pirke,
Okay the mission is a bit different than what I read originally.
the main aim of your aircraft is good rental machine, as everything else kind of comes with it.
Making money on an airframe requires finding the sweet spot in terms of usage, which should probably be 200-300 hours a year. Obviously running costs go up with usage but fixed costs are reduced by hour. For a rental machine check the hours on the engines and the airframe before you buy.
In this case, if the SIDs are taken care of a nice 172 or 182 will make an attractive make for a reliable rental machine. Relatively low purchase costs, easier and cheaper to buy and run than the equivalent Piper. Good parts availability and plenty of choice for maintenance, also easy to resell. Largely good to fly and capable. The 182 makes for an excellent machine and while more expensive to run you can offer something different to the local school, the school if renting from you would want a backup or the possibility add something totally different to the fleet, talk to them if they are in the picture but be aware they have their own interests at heart. 2 seat machines can be a bit difficult to rent as schools quite often have sufficient to cover whereas the flexibility and comfort of a 4 seater makes it a bit more attractive. That said, purchase and insurance is quite a bit higher.
Robins are lovely machines to fly but potentially some pain in finding parts.
Pipers are all basically fine if a bit more limited than the equivalent cessna.
The main headache with low wing aircraft is the oleos on the main gear, the seals have a habit of rolling and can be a pain because they always fail on aFriday night
.
Making money from light GA is hard, lots of competition around and for the return it is quite a lot of investment of time and money. If you expect to be able to cover the cost of your own flying I think you will be disappointed but as you say, I don't know the whole picture but I agree with Foxmoth making a little money from an airplane, usually starts with spending a lot
Good luck