Lesley, I've been thinking about your requirements with the steep return hill and all. As long as your trip home is all up hill, or up hills and flat (no up and down hills) then it doesn't really matter how steep it is.
All that matters is the difference in altitude from start to finish and the distance traveled. Actually it's the direct horizontal distance, but the actual distance going up the hill will give us some extra room for error. So if you can find someone with a GPS measure the altitude at start and finish and then measure the distance. Then decide what the minimum acceptable speed is to make the trip home. So now you know speed, distance, and change in altitude you can figure how much energy it takes to move 100 kilos from work to home. For now ignore aerodynamic losses. Now you know how much energy it requires to move 100 kilos, start comparing different types of batteries. Figure out the weight in batteries needed to provide that amount of energy. If the batteries weigh 50 kilos or more, they will NEVER work. If they weigh less than 50 kilos then you might be able to build something, if they weigh 30 kilos or less than you have a winner. If Lead Acid doesn't work consider NiMH. There are some companies in China making EV size NiMH for reasonable cost these days and the price will only go down in the future as production levels increase. They aren't cheap mind you (not compared to Lead), but they are within the reach of ordinary mortals. If NiMH won't work, plan on getting a hybrid, waiting for fuel cell vehicles, or perhaps buy a diesel and run it on vegetable oil or bio-diesel or some other renewable fuel.
