I just received a call from my dealer on the 2017 Sprinter 4 cylinder that I had ordered in April. It is a "no go" but they offered to change my order to a 2017 6 cylinder model for no extra charge (waived the $1500 6 cylinder charge). They can apparently also order 2018 Sprinters now but only in 6 cylinder models. I see the MB website no longer lists 4 cylinders as an option.
Who knows what the 2019 new engine models will be but it looks like the prior 4 cylinders are history (unless you can figure out how to cut the EPA locks on the storage area in South Carolina where the stockpile of 4 cylinders are probably sitting).
I'd go for it, especially if you are going to upfit the vehicle, as the extra weight could benefit from the extra power. Is the 6 available in both 2wd as well as 4wd? If he's offering you a 2wd 6 cyl., I'd ask him to throw in the 4wd also for a huge discount.
FWIW, we have a 2016 24 foot, 3500 medium high roof 2wd Sprinter that SMB converted last year. We ordered the van locally for drop shipment to SMB Indiana. It has the V-6. Both SMB and our local dealer recommended the V-6 in light of the weight. It weighs about 9,400 lbs with full fuel and fresh water tanks and fully loaded with food, clothing, golf clubs, etc.
The issue is that the 4 cylinder / 7 speed transmission combination is generally considered the better Sprinter engine. It has a little less horsepower but is otherwise smoother, quieter, has better mileage and is more reliable. My Freightliner dealer rarely orders the 6 cylinder engine for all his business customers. They almost all prefer the long term reliability of the 4 cylinder engine.