Yes, hardwood floors can last for generations; real estate agents agree homes with wood floors sell faster and for higher prices.
Slabs are almost always sold intact. Buying random slabs would be analogous to buying fabric remnants. Like a tailor, your fabricator buys the raw material and bases his price on the complete fabrication and installation of your job. The number of slabs and the amount of resulting waste is determined by the size and configuration of your countertop. The fabricator will lay out your job in a way that minimizes waste and maximizes the natural beauty of the material you selected.
A standard counter typically costs substantially less in solid surface material. Quartz, Dekton and granite have about the same price range and are generally more expensive than a solid surface counter.