So I want to start my first breeding project, I've yet to make my design but I did read up on how the whole genetics work. My questions are more practical-stuff related. I would like something 6 vis, so that's what the questions are about too.

How much stable space will I need? How long will it take (on average)? How much can you sell the final pets for? How much gold would it cost?

The answers for the questions you asked can vary a lot. Personally, I use one stable tab for each of my projects, with the top row for males and the rest for females. How long it takes depends on what sylesti you start with, how much room you use, how dedicated you are to breeding, and a lot of luck. I'm about to complete my 6 vis project with themed pets from the easter event and I don't feel like I've been particularly lucky or unlucky. Pricing is something I still struggle with, but i sell my non-themed projects for 2.5k gold per trait points, which is cheap compared to a lot of other breeders. If you start with 3 regular essence it's the cheapest to start, averaging around 15k gold per essence, but you also have to feed all the sylesti while the project is in progress (you'll want a stable hand if possible).
I hope this helps give you an idea of what a project involves in practical terms :)