I have been flying the Corona 120 for almost 7 months. I bought a TREX 450 at the same time but until now, I flew the Corona almost exclusively. I also purchased RealFlight G4 simulator. Following is my recommendation based on my my experiences:
1. Keep the Corona. You will not find a more durable helicopter out there. Fly it until your confidence has built up. Yes a Flight Sim helps but there is no substitute for the real thing. Plus the machines in the FlightSim are perfectly tuned. Your real bird more than likely is not.
2. There is still plenty of OEM Corona parts to be purchased. Mike Roffee at
http://roffeetvhobby.com/ carries a good inventory at a fair price. Very nice guy too.
3. Do not buy aftermarket parts. Yes they help but keep things simple and cheap for now.
4. I purchased a Spektrum DX7 transmitter for the Trex and use it also for my Corona. You just have to buy another 2.4GHz receiver. The good thing is that you will reuse both of these when you upgrade to a new machine.
5. I use 3S 2100mAH lipos instead of the NiMH. Flight times last longer and the batteries can be used on the more modern machines (like the TREX450).
6. I also invested in a used FMA CoPilot. This is a cheat for hovering, but I mostly use it as an "oh sh*t" switch when I get into trouble.
After 7 months, I feel realy confident with basic flying manauvers. I am only now flying the TREX 450 to get into 3d. The TREX 450 is easier to control and is more stable IMO because its a Collective Pitch bird. However it is small. I am now looking at the Century Swift 16. Lots of good reviews, plentiful and cheap parts, and is a good size (more stable in the wind and easy to keep track of when flying).
If you want to buy a new machine. Go with the Swift 16, otherwise I would stick with the Corona.
If you dont want it anymore, I will buy it from you!
PS - Your GY240 is perfectly fine as a HH gyro. That and the GY401 are still widely used and are known for not drifting much.