SCSI drives are not expensive any more - not the used ones on e-bay.
You don't really need anything above 36Gb for the OS drive, and thus they are easy to find a decent prices
Servers will run sata or IDE drive anyway, so you could always swap over to that in the future
The server could be noisy, ours here at work are quiet while cool, but the fan is noticable when it is up to speed.
One thing to watch is expansion slots. If you have an older PCI card to use (say a 5V PCI) it may not work if the server only supports a 3.3V PCI.
Memory is typically more expensive as it is ECC
Otherwise there is no real reason not to use them, they are quality components that work well together.
Mick