Looks like a razor-blade glass scraper and some time will get all that paint/glue off. The tape I would definitely remove, might be hiding a big crack or something.
On the sand/gravel thing I'd just measure the depth inside the tank to what I want. If you're gonna have live plants, 2-3 inches sounds about right. make sure you really wash the sand though. Otherwise you will have cludy water every time a fish stirs it up. Good luck, seems like you got a good deal. Oh yeah, for the top look for a hood at the local 2nd hand store or in the paper. A hood with a light would really spif it up.