DON'T waste money on a formal CAD programme Hover Tim, you'll NEVER use it to it's full extent, even if you're in the drawing "Trade". Have a google for free programmes and there are plenty about, I'm using one called "DoubleCad XT" which is a freebie from the successors of Turbo Cad. VERY much like AutoCad and yes A/C is a heavy programme to use.
Regards Ian.