Hi Nick; don't forget that you'll need some substantial speakers to go with the Studiomaster; Peavey Hisys with an appropriate rating in good nick will set you back £250-350 a pair, plus stands and leads etc. A 'buffer'; I think Richard means a DI box, and you'll need one for each instrument with a piezo pickup; say £25 apiece. (Jump in if I've got that wrong, Richard!) Add that lot up, and you're into HK territory anyway, and you won't get anything like as good a sound from the Studiomaster, especially for vocals. With the HK you'll only need to add a mixer (Alesis, Peavey,etc are good, and Behringer are mega-cheap.) Get one with built in effects, should pick one up from ebay for around £100. It's a really bad idea to try to economise too much on PA gear, as a poor sound can cost your reputation and future gigs. If things don't work out you can usually sell it on again; I sold my seven year old Studiomaster/Peavey rig for exactly the price I paid for it, and the buyer got a good deal. Don't forget decent vocal mikes either; the ubiquitous Shure SM58 would be my choice here.
Hope this helps, if not with the bank balance!
Tim