That's right, the RTR needs extra parts to convert to mid-motor (not many parts by the way). However, I'd stay away from the RTR because of it's first version DIMEC chassis, gear diff (ball diff is the best option in most conditions imo) and the fact it's RTR: Somehow, I've never had an RTR that had decent quality plastics and no overtightened (or wrongly assembled) parts. Build it yourself and you'll learn about the construction of the car on day one (really helpful for doing quick maintainance) and you'll be in control of how well it's been put together, too!
As for the best place to get one... I would say check the stock+prices of MBModels, JEModels and InsideLineRacing. I've had great experience with these stores!
I also found Durango's warranty/support to be quick and very decent, so if you have any questions that are related to quality or warranty you can also directly contact Durango.