I'm afraid I've got nothing for you. The only suggestion I can give you is to try to take the pool back, while you can.
First, the cost of steps that can support 400 lbs is going to be more than the pool. A LOT more. There are therapy slings and steps that would work, but they run $1,000's . . . AND they presume a rigid concrete deck for mounting. No standard consumer above-ground steps will support that sort of weight and instability, adequately. They'd break, or wobble, and you'd fall.
It would be possible to make some adequate steps out of treated wood, but there would be a number of complications, including pool-water-compatible fastener selection, and the need for padding to protect the liner AND a wide base to maintain stability. It would take a skilled carpenter to both design and make such a thing.
Second, the cost of operating the pool is probably going to be more than you realize. Even with the "BBB Method", which originated here, the cost of chemicals PLUS water PLUS electricity PLUS test kits will be over $300 for the season, and probably over $400. That's AFTER the purchase of the pool.
I truly am sorry. I am old and fat myself, and exercising gets more problematic as both age and weight go up. My wife was a water aerobics / therapy instructor for 10+ years, so I'm well aware how helpful a pool can be in your situation. Perhaps there is a YMCA pool in your area, with a therapy sling or ramp steps? It might be worth checking.
Good luck!
Ben / PoolDoc
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