Two options come to mind. A wooden panelled door (maybe with a pane of security glass), or something metal-framed as sold by double-glazers. The former is much cheaper - particularly if the frame is in good nick and doesn't need to be changed - and can be done by any old builder (or even DIY if you're up to the challenge). The latter has the ability to look much cleaner and may be more secure out of the box (I'm thinking of those fancy multi-bolt locking systems, though you also have options with a trad wooden frame).

FWIW we were just quoted NZ$3k to replace a wooden patio door+window combination with an aluminium framed double glazed sliding door unit, I'd expect it to be more in Blighty.