Buy a pre-built Windows Home Server. The HP version retails for $600. Add a second 500GB SATA drive (about $100), and at least a pair of 500GB USB external drives for making backups. It wouldn't hurt to throw in a UPS, either.
Tell the Windows Home Server to set up the two SATA drives in redundancy mode, so your server will stay up even if a single drive fails.
Keep one backup drive attached to the Server (for making ongoing backups) and keep the other backup drive offsite. Swap out the drives weekly or on whatever frequency you feel comfortable with.
If you do EXACTLY what I describe, your odds of losing significant data to "typical" disasters is awfully close to zero. If you add additional external backup drives, you can make periodic (monthly?) "Archival backups" of your data, so you have older versions of your documents available, too.
The WHS will give you redundant file storage, a central place for your files, remote access to the server and to the client PCs (if the clients are running Windows XP Pro or Vista Business/Ultimate, and automatic, ongoing backups of all the client PCs in your office. Note that this last feature is VERY important, since it's costly to restore a client PC that's had a hard drive failure (common) or that's been contaminated with spyware, trojans, worms, etc. With the Windows Home Server, you can restore it yourself in an hour or two.
WHS will do all that you are asking for. If you need more than ten connections or or some advanced features, then Windows Small Business Server 2003 R2 would be good choice. But the installation and maintenance is much costlier and not something for the non-expert. Once you get into the "true server" area, you typically include ECC (error correcting) memory, hardware RAID arrays, and server operating systems.