I was recently discussing the hardware requirements for a new firm with a colleague and we distilled the bare minimum hardware requirements to: a laptop with at least a 15″ display, a black & white laser printer, and an external USB hard drive at least as big as the laptop’s internal drive.
The laptop should be a desktop-replacement class machine. Leave the ultra-portables for another day, this machine is going to be your office workhorse and should have the computational “horsepower” (CPU speed, system memory, and disk space) to be able to run multiple applications simultaneously, and should have a full-sized keyboard so you can type on it comfortably for long periods.
In terms of print quality, speed, and longevity, single function, black and white laser printers shine. It is even possible to find black and white laser printers with auto duplexing, multiple paper trays and support for both envelopes and legal paper in the sub-$500 price range.
Combine an external USB drive and an on-line backup service like Mozy or Carbonite and you have the bare minimum for a redundant backup system. Use backup software like Retrospect to create a mirror image of your system disk on the external drive will provide immediate access to your data should your system disk pack it in. Regular incremental backups to an on-line service will allow you to keep a copy of your data in a relatively safe off-site location. It may take longer to rebuild from incremental backups, but at least they will be there should a catastrophy wipe out your office.