1 - It really doesn't matter. Use what fits in your ride, fits your budget and does what you want performance-wise. Personally, I think small form-factor desktop hardware is a lot easier to work with.
2 - No, you don't need to use a SSD. They're superior to a mechanical HDD, to be certain, but not necessary.
3 - I highly suggest spending lots of time reading the FAQs and learning how to accomplish the goals for your install.