Thanks for those who have offered advice.

There is a fair bit of info on tumblers on this site already - just search on the word tumbler.

My motivation is to build one myself and in my research, mainly internet based, I'm beginning to form the following ideas. 

They come in either a vibratory or rotational form.  Vibrating ones have a sort of bowl on top with a flat lid and the rotary ones are a sealed tube that rolls around.  The general feeling, mainly formed from gun owners (and I'm not prepared to argue with them) is that the vibratory one works faster, but are more noisy and sometimes wanders around on the bench.  Tumbling times seem to vary from about a few hours to over 24 hours.

Gun shops seem to be the most likely place to buy one, some of the other threads on this site mention specific models and Aust mail order places and about $200 seems to be the going rate.

Mediums used are crushed walnut shells, corn cob husks and there has been the odd mention of using brown rice. Pet shops sell walnut shell and corn husk as bird or reptile cage liner.  The feeling I've got is that you fill the chamber about 2/3 to 3/4 with medium, maybe more with a vibratory device.  There has been some suggestions of throwing something called a dryer sheet into the mix which acts to collect the dirt and keep the medium cleaner and longer lasting. I haven' seen these dryer sheets but have not looked for them either.

Additives are suggested, normally about a teaspoon or so, and suggestions have been Flitz a specific additive for tumblers although others have suggested using liquid car polish.

I am continuing to muck around with gears, belts bits of duct tape etc and will post if I can come up with something serviceable - I seek not only some fun in building it on the cheap but also something that works and lasts.