Ramsgate station is probably the closest, around 10mins by taxi? Thats on the line to London.
Geographically Manston is always going to struggle. If you go East, South or North, theres very little catchment area except water. The further West you go, the closer you get to Gatwick and Stansted. You've also got fast rail links from Ashford to Brussels, Amsterdam and Paris to contend with.
So who is going to fly passengers there other than those mentioned? Stobart maybe? Eastern?
All I can think of here is that there is already some cargo comapny involved who will put a lot money into this for a small hub. I dont know what state the terminal is in, or if any of the lighting, nav or ATC stuff is there and working, or even a fire station but the investment needed would be absolutely huge. Thats before you start thinking about taxiways and runway surface.