I have great success using older G1-G2 Compaq/HP servers; ML530, DL380, ML370, ML350. P3 & Xeon systems.
These are affordable even with a very moderate budget and come with hardware SCSI RAID, ECC RAM, Redundant PSU's and often with a DAT.
The Xeon boxes work particularly well for heavy loads; my ISP uses one as his primary mail server which handles mail, spamassasin, clamav & a coupla other authentication modules under Whitebox Linux. This does all its work using about 3GB of RAM, 2 * P3 Xeon 933 CPU's, & RAID 5 on 4 * 73GB HDD's; spikes to 80% load ocassionally but generally sits at 50% whilst dealing with about 3500 users.
HTH