The issue is- "who will I rent it to?" I have a rental store in the mountains, so I'm a prime candidate to you I guess...but I can't see a market. Homeowner? Who's going to pay to rent it to finish mow a slope they can't ride? They weedeat it or let it grow up. Commercial? Maybe for one off jobs... But they'd be infrequent so youre talking very high grass...So if it's to be done often, all them rentals get expensive.
Not to be a pill, I'm not all negatives! I actually think its an intriguing product, you saw a problem and made a solution. But in my opinion, it's a machine the be owned, not rented. Commercial guys who have a steady account with a steep slope the aren't comfortable to ride each week, that's your target audience, as I see it. A homeowner willing to part with that much cash for your machine would be, I think, uncommon. if they have that much cash, they probably don't concern themselves with cutting their own grass! So, the commercial cutter buyer is your guy. So... Now you have to get by skepticism- who are you? How much? If I buy it whats my back up? Will you be around in 5 years? It takes a leap of faith to purchase a high price item from an unknown quantity. So...ever think of hawking the idea to one of the mowing big names? That would make the curb appeal jump up right away.
Again, great idea, and I hope you have the success you want!