I hesitate to approve of any of the "PA in a suitcase" things as usually they sound TERRIBLE. Possibly the worst is the Fender Passport, and the least bad the Peavey Escort. I used one of the Phonics box amps at the Tolpuddle festival one year and was agreeably surprised by it, and it had more punch than any of the suitcase things. Then you will need speakers and unless you are prepared to build your own I think Carslbro are a good bet - but I tried some Altos the other night and was quite impressed since they are usually cheap.

If you use an acoustic guitarist, fine, just plug him into one of the HIGH impedance inputs on the box amp, but if he is a plank player with distortion pedals make him bring his own amp as the distortion pedals will destroy the tweeters (little high frequency speakers) in the PA speakers.

Shure SM58 are a viable option in the USA but in the UK are way overpriced and AKG (NOT the D3800) are often way better, pound for pound, mellower and more articulate.   I tried a Sennheiser last week and it was pretty good. However, even if a Shure soulds like dogpoo (which usually they don't) you will never get any flak for using them, because they are the standard.