I don't have any IBM experience, at least in a Wintel enviroment, but if those servers are anywhere as good as some of their higher end AIX stuff I don't think you can go wrong, but that's a big if.
Past that I've been using Dell servers for the past 5 years and I have not yet had a complaint, they are dead on reliable hardware wise, and out of those 5 years and probably a dozen or so servers in various companies I've only had one problem where a machine locked up and wouldn't boot, spent 15 minutes on the phone with Dell and the problem was solved, past that I've had zero hardware issues. Also, check out their "refurb" side, I recently purchased a couple of servers from there to give them a shot and I have to give them the same high remarks, and there are some REAL deals to be found on there, and you can get the same level of service on those as well (say a 3yr/4hr onsite contract), for instance the last one I bought was a dual 1.2Ghz, 1GB RAM, 4x18GB RAID setup, 20GB DAT tape backup in a 2U rack setup for around $3k (this was about 5 months ago, so adjust price accordingly).
As to how much server you need, well with 2-5 clients and not much exapansion I don't think you can buy a low enough end machine to cover that. I've seen 40 client networks run off a single 500Mhz server with 512MB RAM with no issues at all, other than the disaster side of it (machine fails, you could lose the AD and then you start over). If the applications are small, I'd seriously consider getting 2 low end servers, first would handle AD and be the master, it would also do the regular stuff like DHCP, WINS if you need it, and maybe some file storage - the second machine I would run as your SQL machine and the second DC in the domain. Here you could get away with two $2-3k machines instead of single higher end machine and have a little safe AD enviroment.
Andrew