Yes, it'll be nice to not pay to MOT or Service the car! As for the trade in, as long as it covers a nice bit of deposit on the new car then that's fine isn't it
You'll never make money on a car, but I have managed to work my way up the ladder by buying cars that had already done the majority of their depreciation, paying off the loan to own them and then having a little more ready cash through selling for the next one and so on until a deposit for the FK8 was basically selling the Mugen.
Kinda makes sense...!
Made a couple of K on the RS, bought Elise at 8k sold for 11.5k so yes you can make money. How ever withe Elise after you take into account insurance, service etc no profit.