Consider buying used. A lot of older gear is of excellent quality and will go on forever. My amp, a NAD 3130 is approaching 30 years old and it works perfectly.

Buy a second hand 5.1 receiver. Hell buy one new if you can find something this cheap

Buy 2 pairs of bookshelf speakers. The cheap option is a pair of Acoustic Solutions AV-20. They're cheap as hell but really not that bad. They're by no means awesome but when you see how much they cost... Or you can go higher up and look at the Wharfedale Diamond 9 series of speakers. I have a pair of 9.0's that I got for cheap on eBay. Best £50 I've ever spent. If you're not sure whether the brand is any good Google it and see how many forums like HeadFi and What Hifi it shows up in.

As for a sub I don't really know of a cheap-ish way as I've never used a sub myself. Have a look for active subwoofers on eBay. I'd say that as long as it's of a reputable brand and 150w or more it should match up fairly well. Just don't get anything that's designed to be put in a car. That will sound like donkey. Something like this would do nicely. This one is a UK guy though so have a look on your version of eBay.