You will certainly feel the difference going from your Zetec-S to an RST in terms of reliability/economy etc.

If you get a good RST, then you will be fine, but if you don't then you will have a world of grief.

It's well worth looking at a few and not just buying the first one you come across.

£3500-4000 ought to buy you a nice example, but don't take it for granted.