Sky Watcher now make a complete, all in one auto-guiding unit for £249. I've seen results from it too, and it's quite impressive, but it needs a mount with a RA4 guide port. The most commonly used guide scope is the ST80, and they're only about £80. To auto guide however, you need the Synscan GOTO upgrade for the EQ3 as that's the only system for the EQ3 that has a guideport, and that's another £300. The EQ3 is barely adequate for long tracking though, as the motors are pretty poor, and periodic error will be a problem. It's max load is only 9Kg too, so with a 150P and a ST80 guide scope, it's on the limit. I'd consider a EQ5 as a minimum, but preferably EQ6, because if you're getting into that kind of money, you want some room to develop a little. So the guiding part can be sorted with a Skywatcher Synguider, and a ST80... around £320. But as the minimum mount requirement would be the EQ3-2 Synscan GOTO (£540) you're still blowing most of your money on the mount, and one barely adequate for the job. I'd invest in a EQ6 if I was serious about all this.