Posted: 08/09/2013 at 22:53
Caterpillar tracks was the first thing that crossed my mind, but they tear the place up when you turn, because they're dragged sideways. I'd actually think your best bet would be something like this for a chassis ... ah ... scaled down, obviously. It steers with and drives all 8 wheels.
Other things that might help: feet sprang to mind, as in hydraulic arms that swing out and put down feet to stabilise the thing, but then I thought that people with mobility problems wouldn't be leaning over all that far anyway so it probably doesn't need them unless you're going to build a 1/5-scale JCB digger arm on it. What might be very useful is a tool rack, or rather a set of tool racks for different sizes of tools. I'd suggest checking out some gardeners' tool collections and then building racks to hold those.
A frame on which a spade can be pivoted could also be a good idea.
Oh, yes. Buckets. Taking cuttings requires a rack for the plant pots, a bucket of water and a supply of compost. Something that can hold a sack of compost may be a good idea, preferably with a lid so it only has to be put in once.
The tricky part there is that a gardener won't want to be taking that sack of compost with them the whole time. Ideally, this thing would have a garage in which it was parked, with all its add-ons parked in their spots around the sides, and one could drive up to the compost box add-on, connect it to the vehicle, fold up its feet and drive off with it, use it, drive back in, fold its feet back down, disconnect it and drive off without it.