Ask HN: How to validate an idea? What to include in the email?

I have an idea about a very simple (de-bloated, nothing to install) booking reservation system for small hotels and guest houses

There are people who manage these businesses with crappy sites and I want to offer them a cloud solution with almost zero modifications to their website (just a js tag or a subdomain).

The price? just a few bucks per month.

But, how to aproach them? At first I thought I should send them a personalized/friendly email, 1-to-1. But: should I include a quick demo, flow, screenshots...what?

Thank you.