Just some thoughts
I wouldn't go for the fancy panel just your straight 6 plus an AH on the instructors side. VOR/ILS and a panel mount GPS. With Two VHF's which are stand alone and not linked to the GPS. You can have a fancy engine panel if you so wish. Although a big screen which could display the 6 then be easy configure for EFIS style would be acceptable and being able to turn off individual instruments of the 6 would be a bonus.
The controls would also be designed for hand rudders/brakes etc to be easy fitted without and engineering sign off.
Power plant as per IM.
Metal skin please for the aircraft nice and easy to fix as you say.
Design
2 doors
low tail
low wing
Trimming a proper trim tab on the elevator.
Certified to Spin.
Limited aero's but nothing special.
Sprung steel gear like the PA38 and C152. Large tyres, hydralic foot brakes.
Tail dragger option would be nice.
Cockpit ergonmics similar to PA38 but different door design.
Engine compartment must be able to get a good look at the important bits including being able to get at the plugs with out having to take the cowlings off.
Led landing lights.
12V power supply in cockpit.
And a removable rumble strip on the leading edges. ie so when you are teaching stalling you get tons of buffet over 5 knots to prove a point the rest of the time its normal.
Face level ram air fresh air vents.
Footwell air vents/ hot air ducts. (Its important to have both so you can heat the plane but also get fresh cold air in the punters face)
I shall think some more though and add.