Bobbie, there's no way that a shower door over a bath should let water through IF it has been fitted properly.
Also, I agree with jno. Even these days, any future buyer would mark down the value of the house if it doesn't have a bath.
As far as the cost of replacing the bath with a shower, People like B&Q are not really the right people to ask. BUT, they might possibly know of local tradesmen who do this sort of work. You do need someone there to assess everything. It usually only takes a few minutes poking about.