Also make sure that it was originally designed to hold water. Some reptile tanks are made with thinner glass than aquariums and will not take all the weight of water, sand, rock.
If it doesn't leak and it was designed to be an aquarium, I would think that a really good scrubbing with a vinegar solution would be enough to clean it up.
My 55 was used. 50 bucks!!! The previous owner had freswater fish in it and his brother kicked it and broke the back pane, so he took it in and had the back pane replaced and the whole tank resealed. Then he decide to keep an Iguana in it for about 6 months. All I did was wash it out REALLY good with just water and let it bake in the sun for 2 days and I haven't had any probs at all...!!!