SharePoint 2013 Land page that redirect to another SP Site and gather information

We'll enable a extranet for our customers. Every partner will have his own site collection for example : http://ift.tt/1LC0Gk6.

I would like to set up a page located at : https://extranet.com/ and when people reach this page, they are asked to enter their credentials from a form. They'll login in with their email address, and depending on the domain, they'd be redirected to their site sites/partner1.

For the form problem : I just have to modify web.config to enable my user to login via a form that authenticate from Active Directory. I just set mail as "userNameAttribute" so they'll login in with their email address. Am I good at this point ?

Next, I would like to gather their AD informations, "Last set password" for example, and if it was longtime ago (or if it's the first time they login in), I redirect them to a web part page that asks them to modify their password. Also, depending on a specific field in AD (for example, it also can be a correspondance list in SharePoint) I would redirect them to the sites/partner1. Is this just simple aspx page dev where I use "redirect" method ?

I say "simple" but I'm not a dev, so it may be hard for me. I'm here to ask if you guys have already think about that or if you ever heard about such a project. Then, do you see any point that cannot be realised ? I mean "simply" realised, not something that will last 1 year ...

Thanks in advance for reading all this.