I looked at
This Forest River Pageand the Ford Chassis on the smaller coaches have lower GVWR (coach weight) and GCWR (coach plus towed weight) than the Ford Chassis on larger models, and the Chevrolet Chassis.
The GVWR for E350 = 12500, E450 = 14500 and 4500 = 14200
Frankly, I had looked at FR products a year or so ago and they all seemed to be on E450. I thought that was good. Now I don't know if FR changed their specs or I just missed a couple models.
Auto Transfer Switch (ATS) = Generator will take over powering the coach automatically once it's running. Without ATS your coach has a socket that looks like a campground pedestal outlet. You have to physically "plug into yourself" to get power from the genset. This is no big deal and ATS are not bulletproof trouble free.
Running Boards - Look at the "HOME" view of the brochure linked above. There are pix of three coaches. The one on left (light colored) does NOT have RB's but the two on the right do. It seems that RB's include a "fender flare" (look at the front fender wells in the pix). If you can get into and out of an ordinary full-size van the RB's are not necessary. See what the option price is and what they look like. I just looked at Amazon and aftermarket running boards ran around $300/pair.
Front End Cap - Replaces a bunch of screwed/caulked joints in the Cabover Bunk/Entertainment Center area with one molded piece. This is worthwhile. Deleting a forward-facing window in the cabover is a good idea too. Looks like FR thought the same thing, since at least the pix do NOT show one.
Large Gas Tank on E350 - Available as Option or Standard on certain model cutaway chassis, depending on year.
E450? - Some manufacturers will allow you to "option up." You can always check.