Why is there still no patch out for Dawnguard on pc, or does the pc not matter anymore?

I only have 1.5 update. Is there an update I don't know about? If so, how do I get it?

The current patch is 1.7:
http://www.uesp.net/wiki/Skyrim:Patch#Version_1.7

I don't know though why you can't download it.

Even with the current patch, I still have a couple of bugs, one of them game breaking. Though it's possible - however, unlikely - that it's due to a mod.

Perhaps its due to a mod indeed. However, I'm afraid I can't help with that as I'm playing on the 360

If you give some more details perhaps someone here might be able to help you. Also, check the wikis, they have documented pretty much all the bugs by now. Good luck!

I didn't get 1.7 at all

Steam should auto-update the game, right? Make sure you haven't set it to offline mode or something.