Normally, Craigslist with include surrounding areas, but also cargurus, and searchtempest, but for me the keywords are the "key". Since I have a Chateau package, rare, factory built, and decent quality with quad seats, I search Chateau. Later, Chateau was dropped, then became XLT Premium package. Also since I'd rather have the V10, I search those as well. Even Autotrader in a pinch. For me the E-350 Chateau V10 that hasn't been modified is the holy grail of van workhorses, begging for the right 4WD conversion.
Tons of van fans here, but if you want more confidence in the V10, towing, reliability, and longevity, look here for posts by "carringb" under Towing and Tow Vehicles sub forums. He's probably headed toward 400K on his V10 wagon, and most of it heavy towing, and can really speak to the usefulness of V10 and especially wagons (window van) over the standard work van.
http://www.rv.net/forum/index.cfm/fuseaction/listings/forum/41.cfm
Like others have said, you will be kicking yourself until you end up giving away your Sprinter based van, get the Ford V10 forget about the gas mileage, and enjoy the power and reliability of the best kept secret in van life. Remember that tall roof hits low branches...and when you finish the 4WD conversion it's a lot taller-just sayin.
So maybe a Chateau package, V10, get the 4WD conversion, and pull one of these behind it?
https://www.livinlite.com/products/camplite-travel-trailers/CL21RBS.html
Now you have a base camp that you can leave and return to while you are out 4-wheel exploring in the van that you bought listening to wise counsel on this forum (not from me of course). Finish your research, set the plan, pull the trigger, then when you are sitting by the fire enjoying the destination the reliable Ford wagon just took you to, you can ask yourself "why did I wait so long to get this project going?"
Good Luck.