Here is are my two cents:
I agree with the side windows in the cabover. Some rigs just have one, others two. These are nice, especially if there is one on each side, for a cross-breeze. I haven't seen any rigs made in the past few years that had a front window, so buying new, that problem should be taken care of. The exception are windows which are an integral part of the fiberglass like the one on the Trend.
One thing that is good to have is a fiberglass cabover cap. This not just protects the seams from water coming in while doing down the road, but does a good job at dealing with rock chips.
Going with a Ford E-450 chassis is better, because it has a wider stance, more fuel, and has a larger weight capacity than the E-350. It also has better brakes.
Factory installed rear camera is a must. Not wireless, but a properly installed wired setup. Side cameras are nice as well.
I want at least 5000 pounds, preferably 7500 for the hitch receiver. This gives me the ability to not just carry some items in a cargo rack, but maybe even have a small trailer.
Underbelly insulation and heating pads. I live in Texas, and even though the temperature is normally above freezing, there are a few days each year where it may be in the low 30s, and a cold snap every few years. Having insulation and heating pads means being able to take a shower and use water as opposed to grabbing the air compressor and having to winterize, then use a motel or bath-house.
If you are buying used, have multiple people check for water damage, be it bubbles in wallpaper, discolerations, smells, soft floor spots, areas around the cabover, and other signs. Find any? Run... it is easier to get a new engine and drivetrain put in than to rebuild a cabover.