Any kind of scale that can weight accurately in grams and ounces and can weight 5 pounds is suitable. I got a nice one from Amazon that cost about $20.
It's not really possible to give an exact cutoff number for what qualifies as "too fat" or "too skinny", because guinea pigs come in different sizes and some just grow to become bigger or smaller animals. What you really want to pay attention to is what is called the "body condition score". It basically describes how much body fat the animal should have for it's size. One of the vets I worked with would describe proper body condition this way: running your fingers along their ribs, it should feel like the back of your hand. If it feels like your knuckles, they're too thin; if it feels like your palm, they're too fat. Of course, this was advice referring to dogs and cats, but it should be the same for just about any mammal.