The turbo sounds to be in good condition if the seller is telling the truth, for $100 worth the risk.
Yes you will need a T3 exhaust gasket that goes between the manifold and the turbo. Your dump pipe gasket might be fine if it is a metal one, however replace it anyway since it is all apart and the gasket doesn't cost that much. You may need some washers, bolts or nuts if some need to be replaced. Good to hear that you want to do it yourself, it's the best way to learn and you will learn a lot from doing this. Just follow the instructions on some of the threads on SAU and if you have any questions while doing something ask some one to make sure you don't have to pull it all off again. There isn't much that can go wrong so go for it.