In terms of motors and torque, I recommend you look at the Robot Motor Factor calculator located on this site (
http://www.societyofrobots.com/RMF_calculator.shtml)
As for the positioning of the motors (as I assume that's what you mean by implementation), that depends on your project. Though I would always recommend placing the heaviest components of your robot over the motors to get the most efficiency.
In terms of the chassis, that also depends on the function of the robot. Is it just going to be an obstacle avoider with only a couple sensors? If so, you don't necessarily need the most rugged, expensive chassis, as demonstrated in the $50 robot tutorial. That robot only used a little scrap plastic, cardboard, a water bottle, and some elastics. But if your robot is going to be one that roams the house or is more in contact with a more "hostile" environment, then I'd certainly recommend stronger materials, like wood, plastics, even metal. I used to use a lot of wood (still do, on occasion) though now I've started using more plastics and metal, specifically sintra and aluminium sheets, respectively. The sintra is nice, since you can cut it with scissors or score it down the middle and snap it, but you can also shape it easy (stick it in boiling water for about 10-45seconds, bend, then dunk it in cold water). It is also relatively cheap. Some sign shops will give you scraps for very low prices, even free, and you can buy online.
As for mounting motors, you will often find some sort of matching mounting bracket online, and if not, your motor will more than likely have mounting screw holes in it. If that's the case, you can create your own bracket.
Remember though, it's always good to design you robot idea prior to construction. You will get less frustrated, and it will give you a solid idea of they abilities of your robot later on.
Hope this cleared some things up.
