Theres quite a bit of room in the doors of the RT, Ive got some new front speakers in mine and they fitted pretty easily. Are you planning on fitting a big install? Cos if you are, be prepared for everything to rattle like hell as soon as you turn it up!
Generally, RTs stand up pretty well, I dont think theres anything specific to look for. Make sure that all the electrics work, especially the Heater fan (test it at all three different speeds).
I test drove a 1.2RL (4 speed!) before buying my RT and it was nowhere near as good to drive, so if you can afford the extra insurance, my advice would be to get an 1.4.
Rick