Hatch, you'll find that your faxing needs are very few and far between, at least with regards to expediting. Most anything that needs to be faxed will be shipping paperwork, which is almost always faxed right there from the shipper before you leave. And even that's mostly for loads that need the paperwork faxed to a broker (or your carrier if they handle it) before showing up at the border.
In the last 4 years I've had to use truck stop "scan and fax" three times, twice because the shipper didn't have a fax machine that worked and once because they didn't have access to the office where the fax machine was located. I do have an all-in-one printer/copier/scanner in the van, and have used it 5 or 6 times to scan stuff and then e-mail the PDF file to someone, instead of faxing it. I have also once scanned something and then used my cell phone tethered to the laptop as a faxmodem to fax it to someone, but that was more of a novelty than a necessity. I could have just as easily e-mailed the thing.
Once in a while you might get something from your carrier that needs to be signed and faxed back, but you can sign it, scan it and e-mail it back just as easily, or you can sign in and use a truckstop's scan & fax for those extremely rare instances. At $2 a page, it's not a big deal, and you won't have nearly enough of them to justify a van scanning solution.
Having a scanner/printer/copier in the van is nice, but hardly necessary. It has come in handy a few times when I'm at a delivery and they want a copy of the BOL or something and I only have the one, and I'll just jump into the van and make a copy. It's also come in handy a couple of times when I've taken pictures of already-damaged freight as they load it into the van, and print out a picture for the shipper to have, to eliminate any misunderstandings or claims that may be made later about me damaging the freight.
Paperwork is generally scanned by using Trip-Pack or Transflo at the truck stops, but I use the TransflowNOW truckstop scanning software in the van. Nice, but hardly necessary. It lets me scan from wherever I am, like the parking lot of a Walmart, instead of having to travel to a truck stop, or having to deal with doing it the next time I'm near one.
Mainly, it lets me scan while nekkid, which is the important thing. They don't like it much when I do that in the truck stops.