Well i have chosen to become a vampire lord but on the way i started to dislike it. I mean you cant loot bodies, chests, pick up items and there is also the thing about the sunlight penalties. I would like to remove vampirism (heard there is a quest for it), but will i be able to start dawnguard quest again?

Oh and about becoming and lycan. Is it like the vampire lord transformation? Does it remove dawnguard quest? Does it have any penalty like vampires at daytime?

First, to fix it, talk to an Innkeeper. Any innkeeper, they will direct you to Morthal to meet Falion.

Second, you won't be able to continue with Dawnguard as a Vampire Lord if you cure yourself of it. I am assuming you sided with the Volkihar anyway. They will force you to gain vampirism again and you'll need Harkon or Serana to give it back. Sadly, if you truly do not enjoy being a vampire that much, you'll likely have to start a new character or reload a save before you even started the Dawnguard questline.

Finally, the werewolf transformation is the exact same thing. Third person, no looting, no chests, cannot open doors, etc. There are a few differences that should be mentioned though. First, you can only transform once per day unless you have a specific item. Another thing you should know is you do not have the freedom to move in and out of the form like you do the Vampire Lord. If you go into the lycan form, you must wait until the transformation wears off because it has a time limit to revert to your normal form. It also has no downsides. No penalties at all. On the flip side, it pays for that by not really having many passive upsides. You'll gain a 100% disease resistance, but the only benefit to being a werewolf is if you don't mind the form.

Actually there is, but it's barely noticeable. You won't get the "well rested" bonus (+10% skill gain for limited time) as long as you're a werewolf.

Technically I consider that an upside. Though the idea of a resting bonus is nice, Skyrim already levels a bit quick. Also why I didn't take any of the first three Guardian Stones.