Men...
I honestly forget the recommended thread count, but IIRC, it's not AS important as people make it out to be. Google it and you'll see what I mean. I really wouldn't worry about the comforter "warmth factor", either. Maybe they care about that at Ikea since they originated in Scandinavia, but I'm sure you'll be plenty warm enough. I WOULD get a flat sheet. Firstly, you'll almost always get sheets in a set: fitted and flat, and usually pillowcases, too. Secondly, the sheet, IMO, keeps your warmer than the comforter. If I'm too warm in bed, I'll kick the sheet down and keep the comforter on (if I don't kick off both) and I feel cooler than if I threw off the comforter and kept on the sheet. Another good reason for the flat sheet, it's usually between you and the comforter, keeping the comforter cleaner (unless you kick it down like I just said, ha!).
Also, a fitted sheet is not a mattress pad. The mattress pad is the white thing, usually with a little bit of padding, that goes over the mattress. The fitted sheet goes over that. And pillow shams are not pillowcases. They're a decorative thing to go over the pillowcases and just look nice when the bed is made. I've had bedding sets with them and you take them off when you sleep. Those ARE pretty useless, and yeah, so's the bedskirt. They just look pretty.
A duvet cover is made to put over a plain (usually down) comforter. There are also regular comforters that are one piece, but I don't suppose there's any reason you couldn't put a duvet cover over one of them to keep it cleaner, if it fits. That's actually a pretty good idea.