How to validate an app idea using landing page?

I have an app idea that I believe has a real potential. Since I am going to bootstrap it, I want to only invest in development + marketing after idea validation. Following is my plan for validation:

1. Prepare app's wire frames
2. Create landing page
3. Put wire frames on landing page
4. Direct traffic to landing page through Adwords campaign spending $100

Question is what CTAs should I put in landing page to correctly capture users interest which can help in gauging idea validity. For instance, should it be 'subscribe via email address and we will keep you posted on app development and beta launch' OR a 'Buy' which leads to a page with countdown to launch date..OR something else?

Help please!

If you are trying to gauge how many people will sign up, then i would suggest faking it where the adwords visitor beleives they will get instant action.

For example: https://yourapp.com is the landing page which tells them all the juicy details.
https://yourapp.com/join is the actual join page where the user fills out their details.
https://yourapp.com/thanks is the page where they get notified that the site is in beta invite only and they have been added to the list.

You can simply track all this stuff within google analytics to see how different keywords are performing. You can setup a funnel which would be the easiest way, or can actually use the api to add simple analytics events when these different pages are loaded (probably not necessary).

The thanks page can subscribe them to some mailchimp list where you can use for intial marketing if the app becomes real.

The only drawback of this approach versus shopping the idea with professionals for their opinion is that you risk two things: First someone might just see you are in beta and steal your good idea and do the same think. Getting to the market quicker if they have better capital or abilities. Second, you can alienate the visitor who thought they were going to get their shiny new app after singing up but now is just on a waiting list.

Both of the drawbacks are tiny if you only use the 100 free credit to gauge what is going on (although $100 is barely enough to figure out negative keywords). The drawbacks go away when you stop marketing your site as you are less likly to alienate shoppers or draw attention from potential competitors. Another note on competitors, if your app is really cool, someone will copy it. Google Freshdesk vs ZenDesk to get an idea of what you may be in for.