Focus' have decent OE speakers, so 'upgrading them' won't make much difference. It's also pointless fitting posh speakers if you're not going to have sufficient amplification - When speakers 'crap out' as you wind the volume up it's more to do with the amp reaching it's limits, rather than limits with the speakers themselves. As for the sub, it won't sound amazing for that kind of cash, but it'll add more bass - Finally what headunit have you got, as not many OE headunits have RCA outs so you might not be able to add an amp/sub.