Your best bet if you plan on racing is going to your local track and see what cars and classes have the best turn outs. that way you will always have some one to race aginst.
Right now 1/10 2wd buggy and truck are still very popular in my area.
And the 1/8 burshless stuff is growing like crazy.
I my self have a 1/10 2wd truck the AE FT T4 and an electric converted 1/8 Losi 8ight buggy.
A truggy or (UMT) unlimited monster truck is basically a converted buggy that has evolved into its own class. they have larger tires longer A-arms and a longer wheel base, they are not as popular in my area as the 1/8 buggys
at the bigger races you will see 1 or 2 rounds of truggy and 4-8 rounds of buggy
1/10 the 2wd stuff is more popular than the 4wd buggy
good luck and what ever you do go with good brushless esc's and motors and a quality Lipo charger