Usually keeping the same manufacturer for buggy and truggy is suggested as most of the parts are interchangeable. You will not need to have parts for buggy and then parts for the truggy, keeping costs down.
As for kits, if you plan on racing then a kit would be better. Some RTR kits are capable of racing, but at the end of the day you end up changing so many things that it becomes the kit version.
Tekno NB48.4, TLR 8ight 4.0, Serpent SRX8, Xray XB8, Mugen MBX8, HB and the rest are all up there. See what your local hobby shop carry and go from there. Tekno, TLR, Xray, Sworkz seem to be the ones with the latest chassis on both buggy and truggy kits. Food for thought.
Here is a truggy TLR 8T for sale new with nitro/electric conversion
https://www.rctech.net/forum/r-c-ite...oller-wtt.html