I was wondering if anyone had advice on any RC cars that could be somewhat easy to mod and control with maybe an X-Bee device?
How much are you willing to spend?
If you want to go "easy but expensive", purchase a hobby-grade RC chassis. Sometimes you can find these "bare-bones" - they usually come as a kit you have to put together (note: some have a lot of parts and can be mechanically complex), most of the time they'll have a motor and maybe a steering servo, but not much else.
In that case, you also need to buy an appropriately sized ESC (electronic speed control) to hook up to the drive motor, and a battery (plus a charger).
On the other end are complete "ready-to-run" (RTR) kits that are generally completely assembled and have a radio, battery, and charger (simple as it is) - you basically charge it up and play.
In between is everything else - also called "almost-ready-to-run".
At any rate, almost any of these kits (which vary in price - but expect to spend anywhere from about $100.00 USD, maybe a bit less - to several hundred USD) can be easily controlled by an Arduino using the Servo library.
You might also check to see if anyone is selling a chassis or other parts cheaply on Ebay or Craigslist, or a local classified advert or whatnot. Sometimes people want out of the hobby fast, and they let go of a lot of used gear for much less than it would cost new.
If you want to go with "difficult but cheap" - check this thread:
The gist of it is to obtain a cheapo toy-grade RC car (via a thrift shop or whatnot) - and then "hack" it to control the motors - of which there are several methods, from direct control of the on-board h-bridges, to stripping out the control board and using your own h-bridge solution, to emulating the RC control signals and injecting them into the on-board decoder chip.