I'm only 28 but I do have a bad back so I hope I'm in a good position to advise you!
I like your design, it seems like a good way to make the job easier without having multiple long handled tools. I would question the standard shape of the trowel though - the shape used is fine when you are using it as a hand tool, but on the end of a far longer lever you would need to move the end of the handle a long way to accomplish the same result. You could perhaps consider something a little more curved perhaps? Maybe try digging a small hole in soil with a hand trowel then try the same thing with the long handled trowel and see what you think.
Something else that might be handy is a belt to keep all the attachments handy. If I was using them, I'd probably bring them all into the garden and pop them on the ground, so I would need to bend down and pick them up when I changed them. If you came up with a belt (or even just noted it down in your project as a theoretical addition) you would want to make sure that there were pockets to prevent soil going over the wearer's clothes.
I like the storage rack too - good work!
If this is an ongoing project, then there is a gap in the market for a good de-weeder for those of us who can't bend. There are a couple out there, but reviews tend to be pretty bad. Just a thought!
This topic has made me realise how much I miss Design & Technology at school... for my GCSE project I made a wooden box for sheet music complete with dovetail corners and an inlaid lid!
![Big Grin :D :D](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)