Hey Andy. You should really give the above a try, it is as simple as it looks but when your doing it in front of a crowd it never fails to impress them & that usually translates into sales. To do it on the cheap you can use old cereal boxes instead of paper (whatever you use must have one 'shiny' side on which you paint). I managed to get hold of a consignment of misprinted posters which had been thrown out from a printers shop - all glossy & ideal. As for paint that can be had from pound stores so you see you can set yourself up for minimum outlay. A days practice should see you get the gist of the thing. Best to paint on the flat & put a protective cover down. Not good it it's windy & hot weather makes the paint dry uber fast so you have to be quick.
As for prints (from watercolours only) I always paint no larger than A4 size. From then I simply place the original on my scanner/printer combo & do a high res scan. I then use thick decent quality paper to print as many prints as I like. These take two forms 'open' & 'limited' edition. Open edition means there is no limit to the amount you print, I usually set limited edition as between 50 to 100 copies thereafter no more are made. All are signed & numbered (IE 1 of 100) where appropriate. I find that people tend to prefer ltd edition as they feel they are getting something which may have more value.
Give the spray painting a go, you may surprise yourself as to the results & thereafter what it can generate in terms of spending money! If I can be of any further assistance you or anyone else are free to message me.