Thanks for your replies - I decided to opt for 540gsm black Colorplan for this little project, which I've now managed to get finished for my friend Carlos. I've seen some people specify 700gsm, but in my opinion that's just a bit too thick.
I've printed these using a number of different foil colours, and I've yet to decide on the best combination: any opinions welcome!
Mark at ultimatebusinesscards.co.uk suggested someone (amateur) who might want to do the artwork for the guitar (very kindly for free), and although it's perhaps not as good as it could be, I'm quite happy with it. Although if I ever manage to get a caricature of Carlos I'll do a re-print.