if your worried about buying second hand off a forum head down your local track or shop and ask if theres anything for sale, at least that way you could check it over before parting ways with the cash, if you go buggy in theory the weak parts mentioned for the truggy should have a slightly easier time and last a while longer, RTR are good at getting you running cheap and thats the main problem in the "long term" the parts are not as good to get the kit out at the price, for bashing fine no probs but racing is another thing altogether, at the end of the day its down to what you can afford and if thats the RTR then thats the best way forward for you it might cost a bit more in the long run but its a cost thats absorbed over a period of time which makes it easier anyway, if you buy new dont get sucked into the hype that you will need xxx motor or xxx servo etc etc there are very good reliable products priced well that are raceable without spending $1000's.
Good luck