I cannot speak for the 3rd rail F7, but I can for the 3rd rail SD9. Its easy and fairly inexpensive.
The motor is a horizontal drive that takes up the second quarter of the chassis. (IE the inner half of the front half of the chassis) in The drive belt goes into the tank. From there drive shafts go to the trucks (i.e under the floor). That leaves considerable amount of room on the rear half of the chassis.
I bought a PS3 equipped Railking SD9 (on sale, from a Forum Dealer), took out the PS3, and sold the RK one on eBay. I was surprised how much it went for. I also sold the QSI electronics out of the SD9. As a result my net cost for the whole conversion was about $80.00.
You can use all the exsiting lights in the 3rd rail unit. That makes wiring very easy. My guess is you can use the 3rd rail electrocouplers as well (I model in 2 rail so I did not have them). The only thing I was not able to transfer was the smoke unit. There isn't one in the 3rd rail SD9 and there is simply no room for it. However an F7 might have the room. Here are some photos: