1) Yes and no. New or used, all records benefit from proper cleaning. Cleaning not only improves the sound, it reduces the chance of permanent groove damage which can result from dragging a diamond chisel through a plastic groove filled with (microscopic) bits of junk. Much safer to drag the chisel through a clean groove.
2) Wet cleaning and vacuuming. Other methods provide some improvement, but you asked about the "best" method. Cleaning without wetting is an oxymoron. Wetting without vacuuming away the sludge just rearranges the sludge.
3) Cleaner? Probably. Clean? No. The dirt that matters is deep in the grooves. It cannot be seen without a strong microscope.
2) Wet cleaning and vacuuming. Other methods provide some improvement, but you asked about the "best" method. Cleaning without wetting is an oxymoron. Wetting without vacuuming away the sludge just rearranges the sludge.
3) Cleaner? Probably. Clean? No. The dirt that matters is deep in the grooves. It cannot be seen without a strong microscope.