Yes, second hand speakers are definitely the way to go, and I'd get a previous generation AV receiver that is on clearance (the difference in features will be minimal) so you can use the warranty if needed.
For £300, I'd get a budget receiver (e.g. the Onkyo TXNR414) and pick-up a 2.1 set off Gumtree or the AV Forums, adding the centre and surrounds when you have the budget for it. It's more important to have a set-up that you can build on than it is to have surround sound from the start.