1) Starting a "Canon" playthrough... when do you think it makes the most sense for the Dragonborn to follow the Dawnguard questline. After the MQ, before, or somewhere in the middle? I've never played the MQ past the Thalmor Embassy.

2) If I don't start it early I was thinking of leaving the esm. unchecked so the random attacks don't start right away. Any reason not to do this? Could it break the game later?

I started the Dawnguard quest line as soon as i hit level 10, I did start the main questline first by the time you hit lvl 10 you could already have about 1/4 of it done. I strayed from the MQ to do the DG questline then after that i did the Dark Brotherhood questline followed by the Theives Guild ( Made easier by the fact i was a vampire ) After these two questlines i finished the MQ. The only problem with finishing the DG questline and if you go Vampire before finishing the MQ is that Alduin already does alot of damage, tack that on top of the extra fire damage that you get with being undead it makes it just that much more difficult. hope this helps.

I'd say the worst part of Dawnguard is the trek into Blackreach. I like the place, but it's tedious to get there.
So if you have passed that point in the MQ, go for DG.

Lol i know i can never find the route i always end up running up the mountain doing the left-right jumping trick.

The problem is usually the huge Dwemer ruin before Blackreach and trying to navigate the palce itself, but getting there in the first place is lame too.
I almost ended up giving myself the Scroll through the Console just so I didn't have to go there. I restrained myself through, but this time it serioulsy made me tired of the place.

I am afraid of Blackreach, I hope my level 34 can make it. I remember doing it at a low level, it took me three hours real life time to get through it. I finally got to the dark part and ran past the centurion, I looked at it then fired a firebolt at it. I was so shocked when it came alive.

I can only answer the first question.

Entirely depends on your character concept, which you've given no information on.

I started at level 10 since I made a new character for Dawnguard. Haven't had any problems. You can get the elder scroll outside the main quest, that is no problem.

Character concept = Canon, but I guess that's up for debate. This is my character concept:

Male Nord Warrior, Stormcloak Sympathizer and Companion, so plans to join the Dawnguard of course.

nice someone madee a thread already.... so i have my main guy.... done every questline and didnt boost and im 66 and im a werewolf... i plan to be a hunter but if i decide i want to be the vamp lord can i or no?
and i also made a file with a boosted 81 and i think shed be perfect for a vamp... any suggestions on the little info i gave?

I would suggest doing Dawnguard as soon as you are able. When I downloaded Dawnguard, I chose to go through it first with a lvl 40 character that I had and the whole DLC went far too quickly and none of the gear was beneficial to my character. So I started a new character and as soon as I hit lvl 10 I jumped into DG and was able to enjoy it much more and have a good challenge as well.

SPOILERS HERE: Yes you can choose to be a vampire lord with your werewolf character. You will just have to switch from werewolf to vampire lord. And you will have several opportunities to do so. Other than that, I don't want to ruin any details of the quest.

haha thanks and thanks for not spoiling it... ive tryed my hardest not to look into dawnguard so i can actually be surprised with most of it..... and my guy can pretty much [censored] anything but ive been playing on novice(noob i know but i want a challenge so think if i up my difficulty itll be alot harder or will i still [censored]?

lame cencers haha so ill say kill

I was Level 53 when DawnGuard was installed. My character was far to strong and the vampire hunter side of it was far too easy. I suspect reverting to an earlier save where I can choose the vamp side will be just as easy and just as hard to level up to get access to the new perks. So doing DG at a high level will be very disappointing. I would suggest starting DG as soon as possible (Lvl 10) or no later than about Level 20.

OK... thank you all for your responses... I'm leaning towards starting right after DRAGON RISING quest... maybe getting accepted first into the Companions...

My concern is will I leave Dawnguard way too powerful for the rest of the original MQ? From a Canon sense does it make sense?

what if i go to like expert difficulty with a 66 guy? think ill have an easy time still cause i want to be a hunter