Protein
If you are looking to gain weight, I'd say CytoGainer.
If you are look to cut fat at the same time, I'd say MuscleMilk.
You can purchase both at GNC from prices ranging from 30-50$, depending on size.
It is very hard to overload on protein. If you have excess protein, usually, you'll just urinate it out. Too much protein usually comes will just lead to turning to fat, or liver problems, both thoughs two are very unlikely.