Mephiston, Lemartes, and Librarians are all HQ choices for Blood Angels. You don't have Apothecary's unless you mean you have Brother Corbulo then yes Corbulo does count as an HQ. Techmarines are Elites, and Veteran Sgt. are what lead your Assault and Tactical squads.
I play against 2 Blood Angel armies so I know them fairly well. IMO Lemartes is a must! Or at least a Chaplain running with the Death Company... Mephiston is walking death (actually flying) but he is a little pricey. Only use him if you really think he will pay for himself. The two I play against have stopped using him because they got really unlucky with him and in 1 turn overheated the plasma pistol failed the 2+ save then failed feel no pain and he took a wound. Then using one of the powers took perils of the warp and took another wound...
Honestly use Lemartes and the biggest D Company you can. That scares the hell out of me. A 10 man unit of them on the charge. 40 rending attacks at initiative 5 that get to re roll misses... (Can I just remove my guys without you rolling?)
I haven't checked this out but i heard you can get up to 14 DC with the force organization slots available.Referring to your CODEX, you can have up to 10 DC models, unless you have more than 10 valid squads that give you 1 DC.
You get one free for every one of the following units in your army..
-Veteran Assualt Squad
After that you can buy a DC model, up to 10 guys total in a squad, for +30pts each.