[CyberFonic]

Another vote of support for @sunwooz's answer!

One client is about 99 clients short of a minimal validation. But my concern is that there are several other property management SaaS programs out there. A quick google-fu shows that the going rate is around $10 / property / month. Based on that quick competitive analysis, you would need to confirm that you can deliver something that has compelling advantages over what already exists out there.

Another angle is that at the above rates, you'd probably need a 10 clients, each with a 100 properties to generate a good wage for one person after costs. To achieve that you'd need to not only build the app but also sell it. If you aren't a salesperson, then you'd probably need to get a partner who is good at sales, but then he too would probably want a good wage, so you're up to 20-30 client level and so it goes. And these quick calcs do not allow for payback on the development effort and marketing lag.