I started with a ZX5 RTR and with it's 20 turn brushed motor is was great. However there are quite a few differences between the RTR ZX5 and a kit ZX5. The main ones for me were that the diff out-drives were plastic and the drive shafts had plastic CV type joints so as soon as i upgraded to a brushless system the drive train was wrecked front and back. These parts are metal and a lot tougher on the kit car, they are also expensive to upgrade if you do get a new RTR. I have driven a few 4wds and personally would not go for either a b44 or ZX5. I run a Durango now and don't regret changing. Like yourself i am a 5/10 driver at best and would say the easiest car i have driven is the LOSI xx4, but they do need looking after with 3 belts etc. If you do go for a zx5 get a 2nd hand kit car. Even if the diffs etc need rebuilding it is still cheaper than upgrading the drive-train on a zx5 RTR.
An old post which details the parts that needed upgrading if of use
http://www.oople.com/forums/showthread.php?t=17148