RTRs are ok for just starting out, but in the end, going the RTR route usually costs you a lot more in the end. With RTR kits, the servos and radio usually suck and you wind up replacing them soon after you get the buggy. The RTR versions of buggies usually have lesser quality parts on them too; and this causes breakages and such that require you to replace them. I know that at least some manufacturers use 6061 aluminum for the chassis in the RTR models while going with the much better 7075 in their pro versions.

Just be sure to do your research before you buy anything. With this hobby, it is often a case of pay now or pay more later.