I have one particular character that's level 75, and this is the one I've done all of the quests with. It started out as a mage, morphed into a kind of illusion based archer, and for the last five levels or so I've be kind of exploring the warrior skills by using a lot of heavy armored, two-handed combat. I've been thinking of finally trying Dawnguard. I was going to use this character, mostly because it has so much stuff unlocked, and can be played in different ways depending on what I'm in the mood for. Since this character was started long ago, before most of the patches, I was wondering whether if it might be more susceptible to bugs even in a post-patch game, because some of the data is from earlier (I have no idea how patches incorporate into a game).

Thanks!

Some bugs can only be fixed from a new game, because the scripts have already been run (and quest stages set).

However most bug fixes are fine with an existing game. Don't stress about starting a new one if you don't want to.

My thinking is that you'll be better off since you'll have a savegame before Dawnguard that will be free of problems Dawnguard may cause. For example, if you have already purchased Breezehome in Whiterun, you'll not experience the problem that Dawnguard caused with the Alchemy upgrade not spawning in Breezehome. I had to uninstall Dawnguard, purchase the alchemy lab then re-install Dawnguard. Of course, when I uninstalled Dawnguard I lost all items that I had found that Dawnguard added. Luckily, I had found only one item at the time.

I suggest keeping an old savegame of your character before Dawnguard and then getting Dawnguard and making a new savegame.