Limiting your usage of the scanner may not be wise. Scanners with higher resolution produce the best results. Resolution will affect price, but the difference between 300 and 600 dpi is small. 600 dpi is becoming standard. Keep in mind, however, that some older laser printers only print 300 dpi, and a scanner with higher resolution may not matter.
Speed can be an important factor when choosing a scanner.
SCSI is the fastest type of connection, but it requires a SCSI card installed in your computer. You may already have a SCSI card. If not, purchasing and installing one will drive up the total cost of the scanner. Your computer already has a parallel port, and the printer is probably plugged into it. Parallel scanners are routed through the printer and can be slightly slower.