OK, yep, I was harsh and agree that everybody has to start somewhere.
You need to think what will happen if you build up the bottom - will it be too high against the fence? And if you scrape out the top, will it be too low against the fence?
Can overcome the problem of being too high against the fence by building a sleeper wall (nice new oak sleepers, approx £22 each), plenty of timberlok/timberfix screws (6 inch ones approx £25/box 50 and 8 inch ones approx £35/box 50). Level the sleeper wall horizontally at lowest point of garden and build it with a long return on each end. Build it up to however many courses you want to raise the bottom. Then you can either bring new material in to build up the bottom, or drag material down from the top (provided it doesn't end up too low against the fence). To screw the wall together tight enough then you will need a powerful drill/driver, so budget for that.
You can also build a wall across the garden and split it into two terraces if it's particularly steep. If you need it perfectly level then level pegs, as you mention, will help. But with terraces you need to think about how the wheelchair use will get from one level to another.... a gentle slope going across the garden will do it.
As for surface - build a solid pathway and standing areas for the wheelchair (there are nice Indian stone slabs that are good and strong, or use concrete which can be pretty attractive if tamped nicely and broken up with some brick edging and inlays). Or use 'slip resistant decking'. And for the main area use turf - do it properly and it will give a good firm surface and look nice.
I still stand by what I said before though - despite this only being a small project, it still has to be done right so price it properly as it won't be cheap with respect to time and materials, and if poss have someone with you who knows what they're doing. Otherwise we'll end up with the same scenario as a customer who gets an inexperienced person in to butcher a tree, then has to pay an experienced person to sort it out - more expensive than having got someone else in the first place.