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Depending on the size of the tank, and whether or not the tank has baffles, Option 1 may be worth it. Like Kamie said, I'd have it pressure checked first and see what your dealing with. If it's good, and you can remove it from the boat without much effort, I'd do that. Then flush it good with parts cleaner then water. You certainly want to make sure there are no fuel vapors present, then cut it open using a grinding wheel and clean it properly. If you"re still concerned about fuel vapors, leave it full of water as you cut it. A 10 by 10 square hole would allow decent access to get your hand and a brush maybe on a handle to clean it out. Then take it to a welds shop and have them weld a new piece of aluminum over the hole and repressure test it. IMO, done like this would probably keep the cost under $100 and you'll know exactly what you've got.

You could also probably flush and clean it out using parts cleaner like Varsol also if cutting a hole isn't the way you wanna go. Then to get all the old liquid out use an old electric fuel pump and add the hose you'll need to get it all out.