If you think the dealer is reputable and he's not just scamming you for money by claiming the lens has fungus, then I think it's worth getting it cleaned. The very least you can spent $120 to get it "fixed" and sell it for more if you can find a buyer for it.
If you want to get a one lens that do all, the 18-200VR is a great choice. But like Eric said, it depends on your shooting needs. It seems like you were able to get by with just the 50mm, so it's hard to tell what your shooting needs are.
If you need a little more reach, you can get the 70-300VR, but I would still get the 18-135 fixed and keep it around since 70mm is not wide at all.