I would avoid the extremes in both road and mountain bike designs.

The extreme in road bikes are designed to be ridden tucked in and their tires are extremely thin to cut wind and rolling resistance on a hard-smooth surface. Gearing tends to be focused on speed with only a limited low end. Extreme mountain bikes are quite acrobatic and rugged but they can be a pain to ride any distance and quite cramped and their gearing is mostly low-end for acceleration and climbing. Mountain bikes are often so short that it is impossible to mount accessories like cargo racks and panniers without interfering with petal motion and/or steering. Neither style are known for their comfort or ability to carry cargo.

What you need, IMHO, is a hybrid/touring bike. Something that can be ridden in a fairly upright posture for situational awareness. Wheel size should be fairly large. Tires should have enough width so they don't sink into soft ground. With both enough tread to get traction in mud and sand and but narrow enough with high enough air pressure to be acceptably efficient on pavement. A stiff ridge down the center to allow it to be smooth on hard surfaces is good.

Touring bikes typically have provisions and frame length for panniers, front and back if need be, and racks for bulky objects. The frame on a touring bike will be a bit longer to accommodate these while allowing you to ride and petal without your feet running into the panniers and/or racks or steering being an issue.

Moderation in design, raggedness, simplicity, but with fair efficiency is what I would go for. I would also avoid a bike that looks too nice or is obviously expensive. Having other decide they want it more than you do is not good. An ugly and distinctive paint job might help.

I would start with a simple chrome-moly steel frame. Cheap simple, tough and reliable. I would invest in the crank, gears, derailer and brakes. Heresy to racers I would invest in a good stand, one that works on soft ground. I have had luck with a 'frame-stand', a tiny and light unit that flips down and locks the front wheel making the bike much more stable.