This thread sums up the state of GA aircraft available today. The criteria Ghengis stipulated shouldn't that limiting, but there just isn't anything out there on the market which fits all the requirements. Accepting that anything bought will be a compromise
The requirement "a machine I can have a bit of fun in" to my mind means it must be aerobatic. You would also want to be able to go somewhere two up with a bit of baggage. (I don't necessarily mean the wife!). So now we're looking at only a couple of aircraft.
The Chippy is an obvious choice, but expensive to run with a costly engine and lots of TDs. Also, handling is perfect in the air, but prone to damage on the ground. It shouldn't scare off potential renters - they should want to fly it! It doesn't deserve it's reputaion as being tricky. I have sent lot's of ab-initios solo in them with about ten hours total time.
CAP10s are great, but again expensive and the wooden spar ones are getting problematic. The carbon spar aircraft are very expensive to buy.
The Deacthalon is a good choice. Easy to fly, aerobatic, touring is an option and reasonable maintenance cost, especially if it's a standard with a fixed prop. But it's very stable and nowhere near as good as a CAP10 or a Chippy for aeros - albeit inverted capable.
But these all require hangaring. Which leaves the Rallye and the C150 Aerobat. I never considerd the Aerobat as a proper aerobatic mount. More of a spam can that can do a loop and a barrell roll. Tailwheel versions are available in the USA, but I wouldn't consider them. I've never seen the taildragger Rallye before, I think that would be fun. I used to own a Rallye 150ST and it's a much maligned aircraft. It's well made, handles nicely, the ST is aerobatic and has great short field performance - as well has having four seats. But they do suffer corrosion problems, particularly on the tailplane spar. Hangaring them is a good idea.
Not an easy task. Can't you get your students to design and build a suitable aircraft?