I can tell you this coming from working a hobby store, a track and and spending more than half my days of the year at a track somewhere racing, pitting fast guys and helping new guys...
If you are going racing and want to be competitive (Sounds like you do) my advice is to avoid RTR as basically a false economy. Servos will quickly be either blown or replaced for faster stronger ones, engines are good for learning on but you will quickly want to upgrade and they tend to have parts made more economically such as shock towers... so they break quicker and you end up upgrading quickly and if you are going into racing straight away they can easily become a money pit where if you spent a little more in the right place your worries simply become spare parts to carry, bearings and fuel.
If you really want to go RTR... Losi comes with Spektrum, and the engine has a good reputation as an RTR. Associated comes with the best RTR servos so far by review, i don't know much about the engine but i would bet it's pretty strong as far as RTR goes.
Check out the threads on this forum on the cars you look at and see what people are doing as far as modifications go. Also you will find what the underlying problems of the cars are and how to sort them before they happen.
My advice to start on a minimum budget would be to find a good radio system on ebay (Airtronics M8's are very reasonable second hand - more than good enough and have more features than you will use.), get a strong torque servo for steering (9kg plus, i reccomend the HiTec robot servo with 13kg) and a mid torque for throttle. Aim for a good mid range engine such as the Jammin being raved about at the moment. I would even go for the Novarossi 3 port engine which is a sports engine but a great one to learn with and not slow... i won races in my UK tracks summer series that i am leading with one. RB engines such as the S7 are a great engine in the mid range and tend to have very good life span and power. I swear by RB and pretty much everyone that has run one will tell you the same.
One rule of thumb is to never buy a second hand engine unless you are absolutely certain it has been cared for and know the person telling you so... even then... i would buy new so you know exactly where it's at and find someone experianced to help you break it in properly.
Every engine on Ebay has had only a gallon on it or a few races. Don't bother, it's a lottery.
And plus one on Truggy/Buggy as the guy said early on... buggy will teach you finesse and Buggy to Truggy is definately easier than the other way round.