Sometimes. It should be clear in the the product description whether the set is 3D capable or not. 3D sets naturally will cost more than their non-3D counterparts.
Curved sets have better viewing angles. They can also appear more "immersive" if you're sitting close enough to the set, but I don't particularly care for them. Some people swear by them though. Try to see one in person and see if it's something you might want.
Yes.
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Those are due to actual ghosts. They're the vengeful spirits of the laborers who assembled the sets in poor working conditions and eventually committed suicide. Whatever you pay for the television, be prepared to shell out an extra $150-$500 for an exorcism and color calibration.
Edit: In regards to 4k, depends on how much UHD content you have access to. Pickings are kind of slim at the moment, but 4k sets typically only cost a few hundred more than 1080p sets of the same size. I recently bought a 70 inch Vizio M-series 4k set for about $1500 and it looks stellar, but it lacks some of the newer bells and whistles like HDR. It has extremely low input lag for a 4k set though, which was more important to me. Your mileage may vary, but a 4k set will generally be more future-proof than a 1080p set.
