The best way to build an MVP for any SaaS product is to create a landing page that looks like a real product. Here's an example of one I built. http://www.happiily.com

In this case, it advertises the primary features of the product and invites people to sign-up. When they do, they are asked for information which qualifies the person and then sends me an email.

I built this quickly and very inexpensively and started getting inbound leads from it shortly thereafter.

I got on the phone with each person who signed-up and explained the features I wanted to build and was able to do a lot of customer learning based on that.

Happy to talk to you in a call if you'd like to talk more about customer development with SaaS products.