Keep the Mark IV, it's one of the best turntables VPI ever made. I think you would have to spend way above your budget to get better and any of the models you mention would IMHO, be a backward step.
The Mark IV warrants a much better arm. I once had a Mark III and going from a Rega arm to an SME V made a huge difference.
Maybe look for a used Graham 2.2 ..... even a Mk1 Phantom might be had within your budget as would an SME-V. I've seen Basis Vector arms for sale for about $1500 (I bought one!) and at that price you get a real bargain. Basis arms can be a bit tricky to set up on non Basis turntables however.