Dad's right. I'd use what you have until you're in a position to upgrade your whole PA, rather than buy things piecemeal. You may well decide to go for active cabs (once you get into the realms of better kit, many PA speakers are active), which would mean you'd be wasting your money buying powered mixer. Mixers, especially analogue ones now many are going over to digital, tend not to hold their value and you don't get a good resale price if you buy new (I have just sold a large Soundcraft, in mint condition, for a fraction of what it cost me). As a stopgap to provide extra power for larger jobs, I'd look at picking up a used power amplifier to drive the other speaker. If you really want a powered mixer, they often turn up used on the 'Bay. Stick with reputable makes such as Yamaha, Allen & Heath, Soundcraft, etc and you'll be fine.