I have to disagree…I don’t think there’s anything wrong w/ plastic (though I do have some environmental qualms, I will admit). I am incredibly anal, and I still felt perfectly fine with plastic cutlery. We chose black plates and black cutlery. On the other hand, we also were using wooden chopsticks (the rounded, more elegant kind, not the crappy splintery stuff) and miniature "Chinese takeout boxes" and "saimin soup spoons" (um, Chinese soup spoons?) as serving dishes, so there was already an element of "disposable-ness."
For us it also worked b/c we did a "heavy hors d’oeuvres"/cocktail-style service — food stations and passed trays. I don’t think people expect china from a passed tray as much as they do for a formal dinner. We did, however, have real glassware, not plastic. It may be a little stranger for your set up, since you say you are doing buffet. If you are serving meats that need to be cut, that may also present difficulties.
But, to be honest, I don’t think that many people will care, as long as you’re not using super flimsy paper plates with cheesy designs that spill on them. If the food is good, they’ll care more about that and not notice what they’re eating off of. These days, there are some very nice heavy plastic pieces. There are even ones that imitate glass plates. Whatever you choose, just don’t mix and match cutlery and plates (go all disposable or all "real.")
As far as open seating, we did that as well. Again, we did cocktail-style service, so it was not so shocking. It saved us a lot of stress about figuring out who had to be kept away from whom, and who would want to sit by so-and-so. We did save tables for the parents and their selected guests (10-top tables). We didn’t force the bridal party (plus spouses) to sit together; they sat where they wanted to.
Honestly, if everyone has a good time, enjoys the company, the food, the music and loves you, no one will care what they ate of off or if they chose their own seat. You know your crowd best; if it’ll offend them, then yes, maybe you are better off with "real."