I wouldn't say SD is a clear favorite, but it's strong and popular... and cheap. Most of the questions you're asking can be answered simply by watching and reading the threads here in this section. Plenty of people posting about combinations of steroids and intended cycles. Then you have varying posters with varying degrees of intelligence and education that chime in. Some are to be listened to, some aren't.
You can easily check out what is schedule 3 by going to the website. I wouldn't be too concerned with buying SD at this point even though it's schedule 3 now. They're not after random bros buying 2 bottles of SD or phera or M1T. As far as companies to buy from now, many have purity concerns and it's a gamble sometimes. Right now i'd advise for: LGI, Antaeus Labs, CEL, Shredded Labs, and maybeeeeee Hard Rock as a last option. If you want SERM help, use the search function and look at what people are doing for SERMs for their respective compound.
My advice for cycles is keep it simple. Buy 1-2 bottles of your desired compound, 1 bottle of nolva or torem or clomid, some sort of organ assist product (TUDCA for harsh cycles, Cycle Assist for mild cycles), and then something that will help natural T production (forskolin is the top dog here, DAA is decent). That's it really.