"Upgrading" and "budget" rarely go hand in hand for long, so from that perspective it's probably best to limit your replacements to only those parts that are actually worn out. Apart from that there's usually hard to beat Shimano when it comes to value-for-money for utility riding. Nothing wrong with square taper either. wiggle.co.uk and chainreactioncycles.com for instance should have something decent to offer, once you'e determined what bb you need.