1) He will be an upgrade to a regular Marine. All pertinent info for the Sgt. (+points, wargear, etc.) is listed in the options of the entry for the squad in the Codex.
2) Yes and no. You may have as many heavy weapons squads as your Codex allows Heavy Support Choices. In most cases, three choices/squads. For Troops Choice squads, most armies' squads can only have a single heavy weapon. There is generally only one "special" weapon (Meltagun, Flamer, Grenade Launcher, etc.) per squad, however, some squads and armies allow more.
3) Nope. Anyone who demands that you do should get an honorary kick in the jimmy for being a douchebag.
4) In any game, you can take any HQ your army allows, so long as you have the pre-requisite Troops choice.