Is it true that most authorities say that baldness and left handedness disqualify a man for Kehunah? Does the same apply for Birchas Kohanim?
They are indeed disqualifications for serving in the Beis Hamikdash (Rambam, Hil. Bias Mikdash 8:1,11).
This doesn't apply to birkas kohanim, though; there, a kohen is disqualified only by blemishes that are visible and will distract the congregation. (And even then, not if everyone in town is already familiar with him and his physical defects, or in places - as in most Ashkenazic congregations - where the kohanim cover their faces and bodies with a tallis.) The reason is that birkas kohanim is not quite the same as serving in the Beis Hamikdash: they both have to be done standing, but other than that they're not comparable. (Shulchan Aruch Harav 128:44-46)
According to Bechoros 43a-b, (in agreement with Alex)
Leviticus 21:21 says "any man of the seed of Aron the Priest that hath a blemish shall not draw near..." We learn from this passuk that there are two types of blemishes which make a Kohen unfit for service in the Beis Hamikdash: any physical blemish that would render a Bechor unfit, and anything which makes him look funny (or disgusting to look at). If he's funny-looking, then it appears as if he is not "of the seed of Aron", since he looks different than the other Kohens.
A real physical-blemish would nullify his avodah, but if he is just funny-looking and he does the avodah, it's not null, but he is transgressing a positive precept.
If the Kohen has even one row of hair from the back to his ears, but not in front, he is fit (it's a balding pattern which is more pleasing on the eyes than the others), however he is unfit if he is completely bald, has hair in front and back but not on top, or has hair only in the front and not in back.
Lefties are disqualified for Serving in the Beis Hamikdash becasuse: