You are correct. Transfers that are 19 or older at the time they start school can't live in UDA or RCA buildings. (That's a new rule this year, because the return rate for the residence halls is so high; they're operating at ~96% capacity in the falls.)
Since you'll be 19 when you start, I highly recommend living in Buchanan Hall. I've lived there for the last two summers, and have lots of friends there during the school year. It's a great place. You get a good balance of those who like to party and those who like to study quietly, and you'll have no problems no matter which one you like to do. They're a relatively active community, but there's no freshmen in the building (except for about 10 kids in a learning community) so activities are a little more chill. You share a private bathroom with one other room, and floors 3-8 have their own kitchens. (1 and 2 use 3rd floor.) You're right across the street from the MU, 9 minutes from the UDCC, 3 from Linden, 5 from Oak-Elm and MWL, close to Hilton and Stephens, have a bus stop in front of your building and a huge parking lot in back, close to Campustown, etc and so on. It's a great place to live. You won't have any trouble making friends, either.
I can tell you anything you want about living on campus, PM me if you have more questions.