All fine advice above, but if you really want to get a 5DII, I wouldn't worry about it. Whether it's an amatuer camera or a professional camera or a prosumer camera all depends upon whom you're talking to or reading. But no reasonable person is going to deny that the 5DII can give you outstanding results. And unless I'm mistaken, plenty of wedding/event photographers and portrait photographers have been using the 5DII. And, won't swear on a Bible, but I doubt that your average client is likely to look at it and think that you're not using a serious camera. And since you and we know that you intend to shoot studio portraits with it, you might as well also get the power grip for it, so that you can comfortably and efficiently get those vertical portrait shots that I'll bet you and your clients will want. That's likely to make it look even more like a monster of a camera and somewhat like the 1DSes.
About the 1D cameras, I'm sure a 1D camera would be fine, but if you're going to get a used 1D-Something camera, I would recommend that you avoid the 1D cameras, since they are 1.3 crop sensor cameras, and their main claim to fame is high shutter counts per second, which makes them good for sports and the like, but you don't need that for subjects that are sitting still in a studio.
But you want to shoot portraits, so I'd recommend that, as long as (if) you're buying one of these old cameras, you should probably get one of the 1DS Somethings, if you can find a decent one for a decent price, since they have fullframe sensors. Yes, sensor size matters, I think, especially for portraits.
And here I go. It's not like you can't find plenty of people complaining about sensor dust with the 5DII, too. I don't know off the top of my head which of the 1D and 1DS cameras have or don't have built in sensor cleanors, but that's (I think) something to think about in your decision making process. And of course, you might as well already be thinking about how you and your partner need to try and practice good practice regarding sensor dust. For instance, you should always try to keep the camera body oriented downward when putting a lens on. I find that to be almost impossible, myself, but I do strive for it. Anyway, for example, never lay your camera body upwards. It's simple gravity; the dust will fall in. Another thing, get a good, strong blower and always blow dust away from the back end of your lenses before attaching them to your camera body. Etc. The point is, an ounce of prevention...
=)
