Welcome to Truck Conversions! That's a big question. Do you already own the truck? If you don't then I think you might be better off buying something ready to go. If you've got the truck then you're probably right to consider building it. I'm not that good with numbers and such. But, when you come up with what you think might be your budget, double it. That should get you close to what it's going to cost. Then, consider what you think the time line is to build it. Then double that. You might be done about that time. There are some truck conversions available built on sleeper cabs with very nice seating somewhat like what you're talking about. I would take a serious look at this one:
https://www.hawkengineering.com/4509gcddgta0csb.html
That page shows the asking price as $102k. But, elsewhere on Hawk's site it shows an asking price of $81k. I suspect you would not be able to build the coach you're talking about for anywhere near that, even if the truck is all paid for and not counting that cost. And, this truck is built on a much heavier duty chassis with bigger motor and trans than that M2. I've got no connection to Hawk. I looked at that coach pretty close when I was searching and really like it. Hawk said it would pull a 10k lb. trailer just fine. The drop deck garage prohibits a super heavy duty connection to frame for the hitch that's why he wouldn't advise a heavier trailer.
Anyway, this is just my first suggestion. There are lots more. Lets get some more comments here and see what others think.
Budget Estimate: Converting M2 Crew Cab Box Truck
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