Two kinds of scale are useful. The first, and more important, is a digital scale to weight components and materials as you make them. I have one that reads out in 0.5g. Think of this kind of scale as your "reality check" while you're building. Second, you need a way to weigh your finished model. But really at this point, if it's too heavy what are you going to do about it? So don't worry about this end so much. Anyway, for most normal sized models all you need is a cheap fishing scale that you can suspend somewhere. Most are accurate to within half a pound and that's all that matters for an "all up" weight.