Hi Mandotim. Yes - buffer amp, DI box, guitar preamp - all do the same job for this purpose namely to let the piezo pickup see the really really really high input impedance it needs. It MUST be an active DI box. There is a Behringer one you can run in stereo (ie use it for two guitars at once) you can get for about 25 quid.

If your piezo equipped guitar/acoustic bass/mandolin etc have a decent preamp (Fishman is the industry standard) in them, then you will not need a DI box (or buffer amp). I got a belt amp from the USA for $25 for my mandolin which had no preamp for the piezos in it. LR Baggs Paracoustic is nice (but very dear) so long as idiots do not twiddle the slider settings. If they do it can ruin the sound of a nice guitar until intelligence is re-applied, I've seen it done. My camping guitar I put a cheap Shadow piezo in it and cut a hole in the side for a cheap pre-amp/graphic that I got off ebay for £20.

I would NOT use Hisys speakers for acoustic - thud and scratch as I said. The cleanest PA tops I have at the moment are a pair of proprietary wedge shaped tops I bought with no drive units in (cost, £40) with an Eminence Sigma 15 plus two Motorola powercell piezo horns in each. Took a bit of woodwork. Total cost for two 350 watt RMS 8 Ohm tops with almost indestructible tweeters - about £150.

Use separate bass amp at this power level. Hartke are lovely but costly. The little Behringer tries to look similar and is awful.

Do not DI the guitar amp for electric guitar. Mic the cab. The distortion that a guitar amp is designed to produce sounds lethal through horn loudspeakers. The cone loudspeaker in the guitar amp will not reproduce the extreme spikes and mic-ing that will sound a lot sweeter.

HK have got better, but I remember seeing an awful lot of blown drive units from their PA cabs, and heard some very rattly ones still in cabs, at North West Kent College a few years back and I have never really trusted them since.

Mics, Shure are the industry standard but you can save a lot of money and sound just as good, or better, with a carefully selected AKG.

Make your own speaker leads with Maplins HIFI cable and Neutrik plugs.

I think you'll need an amp to drive the foldback off that Studiomaster, one of the old H/H 100 watt things you can get for a tenner or so will do. You'll need a couple of monitor wedges too, the little Laneys are good but you can build your own cheap with an Eminence Alpha 12 and a Motorola powercell.

Carry a lead tester box. The time they save over piddling about with a meter is amazing. Always carry spare guitar and speaker leads.

Oh, yes, the Studiomaster is heavy. That is why it punches its weight sonically (lightweight power supplies are not smooth) and tends to stay working.....