[ModSocks]

And to better answer your questions,

If you're buying a complete tower, the OS should already be installed in the first place. It's best to just buy it pre-installed because buying an OS by itself can be pricey depending on the OS. Windows 7 is about $100 (around there), and Windows 8.1 is pricier still.

Yes, you can buy a refurbished tower w/ Warranty. I'd use Amazon, personally. Fast delivery, good customer support and hassle free returns. That means a lot. I've heard nothing but nightmares about Newegg. You might save $10-$15 buying through Newegg, but i'd rather spend that extra money with Amazon knowing that if something goes wrong, Amazon has my back.

No, you don't have to concern yourself with brands, but like i said in my other post, big name brands tend to have more unwanted shit on them that can bog down your machine. HP is notorious for it. They're the worst imo.

I WOULD however take a look at the brand of components, particularly the processor, for reasons i explained above. Its much cheaper/easier to add RAM, an extra HD or a fancy pants GPU if you need it, than it is to swap a CPU/MOBO combo.