Neon ACR's are not as good as many of the other mentioned vehicles. My roommate autocrosses a 1st generation ACR, I've posted pictures of it on the site before. He personally dislikes it, his previous track car was a 1993 Civic 3 door which pretty much walked all over the Neon. ACR's I'm sure are good at some forms of racing, but they are not as good as other vehicles mentioned when it comes to autocross.
Plus I've never seen, and my roommate Chris agrees, a car that breaks down so much. Yes it is a track car so we both expect it to break down, but not as often as it does. It seems like he would go race it every Sunday and I would have to go tow his ass home or run him up some parts.
After talking with him today, he suggests a Miata for an autocross car, but also likes the idea you are looking for BMW's.
Also McLaren are you thinking of joining the SCCA or are you just going to start with local club sponsored events?