does it thread automatily

Oh no darling. This one takes a magnifier with the instructions. It arrives pre-threaded, and I suggest you take well lit photos of this with the panel open to see how its threaded. This way you can refer to them rather than the instructions, as one of the illustrations is slightly inaccurate. There is a learning curve to this, but if you are patient and follow the instructions along with your photos, you should be fine. The mistakes I've made are from the threads getting hung up at the spools. Attention to detail is a must with operating one of these, and double check the steps. This must be threaded in a specific order, or it won't work. It does work well, but takes some acclimation. Don't get discouraged. If I had the money, I would have bought a Bernina (made by Juki), or something that is easier to thread. I don't use it enough to warrant a thousand, or two, dollar investment. I got this one at a black friday deal.

Will this serge through the thick side seams of denim to hem jeans?

Mine has. I just change my needles for more heavier weight fabrics. I have used my serger to trim the legs of my adult children’s jeans with no issues.

Can you buy additional special feet for this machine. What kind of special feet?. And how expensive?

I can buy special feet like: Piping Foot, Beading Foot, Curved Pearl Foot, Universal Blind Stitch FootThese 8 feet you can buy with 129$

What is the best brand of thread to use with this machine?

It really depends on what you are sewing. I usually use the cheapest cone thread I can find and it works great. But I also use sulky thread when I want to have a fancier rolled hem.