Welcome to the forum, you’ll find a lot of helpful people here.
In answer to your questions:
a) Stacks is a fantastic way to extend the functionality of RW, the install is straight forward. Stacks appears as a new page type in the menu, from there there is a separate Stacks library which you then drag and drop to create your page. Stacks comes with a few basic ones and you can buy many more which can really extend functionality.
b) you can only change to https if your web hosting supports it and can give you a SSL certificate. There are many free options, if your supplier tries to charge you for it walk away and change your hosting company.
For your other questions I’ll try to answer them altogether as they overlap a bit.
You do need Stacks to install Foundry, as it depends upon it. There are a few options, depending what you want to achieve for a new contact form.
As you don’t have Stacks yet you are at an interesting point.
You can buy the stacks you want to achieve what you want to do, another option is to look at the Frameworks available (Foundry is one of them). Different frameworks offer different functionality, so worth looking into seeing what each does.
Individual stacks will deliver what you want, Frameworks are large collections of stacks that cover a lot of possibilities and functionality in a single purchase. As you have already been looking at Foundry I’ll leave it off the list (not one I have, so can’t offer much insight on).
Frameworks to look at are:
Source by Shaking the Habitual
Free basic version and not so expensive full version (£20). A micro framework which focuses on layouts, images, text; doesn’t have it’s own contact form, sliders, etc. You can do a lot with it but may need additional purchases to achieve what you need. Paid training courses are available, as well as project files, to get you up and running faster. I have done some of the courses and they were very good, not too expensive (~£20).
Foundation 6 by Weaver Space
$99.95. Very comprehensive stacks suite but unfortunately documentation is thin on the ground at the moment because it’s only just been released (it’s a much anticipated upgrade and the dev behind it, Joe Workman, wanted to get it out to users asap) there are a good few free videos showing how to build and functions available. There has been some significant upgrades to it’s predecessor Foundation 1 (less code, accessibility), which was very popular.
UIKit 3 by Weaver’s Kingdom
$99.95. Another one which has been updated this year and proved popular lately. A fairly comprehensive stack suite, unfortunately don’t have it but some have said a bit of a learning curve which once you complete can achieve impressive and fast sites with. Both Weaver’s Kingdom and UIKitter (a Third party) offer project templates for purchase to get up and running faster.
Platform by Barth Partnership
A recent addition to the field, a comprehensive suite. I have it but haven’t gotten around to playing with it much. I think it’s currently on 50% offer at $34.98.
Hope this helps,