I can't comment on a T-Rex as I haven't flown one. I'm new to heli's and I am flying a CP myself right now. I upgraded to LiPO a few weeks back, here is what you will need.

A new main motor with a 9 tooth pinion
3S 800 Mah LiPO (or higher Mah but watch your weight)
A LiPo charger
Main & Tail motor heat sinks for sure.

I did not fuse my 4 in 1 and haven't had problems with overheating thus far. Another upgrade you should definately consider is the Bell Hiller Mixer, I did that one today and it made a world of difference.

All the above upgrades can add up money wise, so consider that carefully. You can easily put over $100 into upgrading your CP, plus what you paid for it in the first place. Weigh that against getting a new Pro for $220-$250. Me, I'm done upgrading for now and will learn to fly better before moving up to a Pro (to add HH Gyro) or a T-Rex.

I hope I've been helpful.