Depends on amount of deposit, affordability, credit history and whether you're buying from one of those dealers that offer special low-interest deals - if not one of these I wouldn't hold my breath for below prime - probably quite a bit above.
Fixed will be higher than linked, generally. You need to consider how long you will take to pay the car off, and whether you think prime will move higher within that period.
I got 9.8% linked last year when I bought my car. Fixed it would have been 14.5%. I went with linked. Even if interest rates go up (which I doubt will happen this year), it will still be cheaper than paying 14.5%. this was with Standard Bank Vehicle Finance.