What about a server like a HP ML350 G6? Fits all your requirements including ps2 and serial ports. Only downside could be cost of memory. They run quite quietly for servers.
They tend to be a bit noisy on startup though. A few of our customers have been quite alarmed when they've heard them :) When cost cutting has been required, we have sometimes used Kingston branded memory for HP servers and have never had a problem with it, and it's often less than half the price. With a brand new system you'd need to be aware that for a warranty call HP may require you to reproduce a problem with the non-HP parts removed, but that rarely comes up. Also, you can get empty SAS/SATA disk caddies and buy your own disks and save quite a bit that way too, although I wouldn't recommend it for the higher end HP RAID controllers like you'd find in the ML 350 servers. I've only done it with the low end servers where I use Linux software RAID (and bcache). James