Once the mixer is finalized, then we can talk about what the computer needs to have, to do what he wants. The real question he will have to answer is - what quality is he willing to compromise for? More quality equals better products which equals more money.
I think you're right about the main things being RAM/Sound Card. But a SSD can help as well if the mixes are complex. Or at least 7200 SATA2 HDD'S. I have nothing good to say about the M-Audio 10/10 line up. I got angry last year and literally tore that thing out of my machine, then smashed it on the concrete. I've been eye balling a E-MU one as of the late. Get 16GB ram as its really cheap these days. An i3 will do the job but if possible the i5's will be future proof for a lot of hard core new plugins that will come out.