RE: "Big plans little budget little knowledge"
Well, from a design standpoint, locking out a chain drive is a big mistake right away. It is so simple to setup. Plus, it's a bit different than your dirtbike, it's a much smaller area, and length, and unlike that, you could place a pair of discs on either side of the sprocket to keep the chain on. Only thing to consider, is keeping the chain tight, but if you place the idler sprocket at the swing point of the suspension, it's no big deal. Otherwise you need a gearbox, or a driveshaft to get the power. Which, can be a mess when making it yourself. Besides, if you are doing a straight solid shaft, a chain is a one step deal.....just put it on.
For an engine, go 2 stroke. 4 is not as good, only because of the power. Well, maybe the price too. Depends on what you want to use, most engines this size are 2 stroke. It's up to you though.
Wheels and tires, use the front tires off a small riding mower, of course it depends how big this truck will be, but for mine, since it's a 1/4 Hummer, I used 10.5" dia. tires. But this is also a 49" long truck, so try to keep it proportional. To big, it's won't turn and you'll spend a ton on a servo, too small, and well, it won't go anywhere.
As far as a radio, any will handle it. It's a question of the servos, and how much you have to spend.
Before you buy the engine, servos, or shocks, you will need to know two key things. How big is it, and how much does it weight. Those two things will determine the strengths you need of those three parts. The bigger you go, the more power and money (or friendly giveaways) you will need.