I do community gardening and there are a few seniors who have problems with bending over to weed their plots. Our solution was to build the plots up to 24 inch height for them. We used 12 inch square timber for them because we were able to get a really good deal on it.

The plots are 4 feet wide and up to 16 feet long. The 4 foot width works for them because they can reach it from both sides. If you could only get at one side, like if you were against a wall, you would likely want to keep it at 2 feet wide instead.

The women who are using these raised beds can get by with very little digging and since we are outside the drainage is not a big problem. From what I saw the only real problem they had was when they grew taller plants than they could reach easily, so you might want to remember that if you are going to plant on trellisses or grow pole beans.

We filled our plots up from the ground to the full height with soil, but if you wanted to you could put containers on strongly built racks instead. The plastic storage bins work well for that, and the racks don't need to be very tall.

There are also plans for self watering planters on the internet that look nice so that might also be an option for you to consider.