You will need a single passenger seat, seat base, and a swivel to go in-between them both. This will cost in the region of £700 - £1000 depending on how lucky you are with sourcing a single passenger.
The double will come off with eight nuts/bolts, as will the double swivel. When you have the double off, you will have eight threaded bolts protruding up from the floor, you only need four. The two outermost (near the passenger door, and of the six remaining, the middle two. You can remove the remaining four bolts with a 22mm spanner. It's advisable to do this as they take up space within the seat base, and the two left in-between the two seats are ugly, and will tear your mat if you chose to have one.
The difference between the two swivels, double and single, is that the single swivel goes on top of the seat base and below the seat so the seat base is fixed to the floor. The double seat swiveled everything.
It is worth it!