I've seen standalone units that record to an HDD in a removable sled with 4, 9 or 16 inputs going in the $400-$600 (wholesale, that is). One of those would certainly do what they need, and even an 80GB hard drive (about the smallest you'll find new these days) would give them at least a week or two of storage. The downside to them is they have no output devices - no CD writer, no USB ports for an external drive or "thumb drive", no flash-card slots... the only way to export video from them is to hook a VCR or other capture device to the main monitor output and just record stuff realtime as it plays back.
I suspect once they start getting used to having that video available, they'll start wanting to review older footage... say, if a daily or even weekly count of something comes up short, they may want to go back and see if some units "wandered off". They'll also find that the ability to NOT have to sit and watch the screen constantly becomes very attractive. Also consider the following scenario: kid rips off something more expensive, gets caught and handed over the police; police come back a week later looking for video evidence. Nice to have that extra storage.
There are lots of other instances where it can be handy to keep footage around longer... to capture a break-in while the store is closed, for example... catch employees skimming from the till... monitor people trying to pass fake money or attempt sleight-of-hand tricks with their payments... many DVRs support point-of-sale interfaces that will overlay a text readout of what's going through the till, over the camera image of the counter, so it's easy to reference later that what was rung up matches what was actually bought.
And that's all just for starters :) Your clients may not be thinking of these things now, but it's some things you should probably run by them for consideration.
Not necessarily... a "SERVER-QUALITY" motherboard will be more reliable and handle data better than a cheap $100 all-in-one board. As with anything else, it's a matter of balancing requirements for reliability vs. cost.
Sound like they're halfway there...