I've been looking at a few bikes, the Trek 520, the Surly Disc Trucker and the Kona Sutra seem to stand out.
Those are some of the most reasonable bikes you can buy today. The optimum material for bicycle frames and forks has long been butted chromium molybdenum steel (chromoly) and with recent "inventions" such as aluminum and carbon fiber it's all been going downhill since chromoly.
If I was about to purchase a non electrically assisted bicycle today, Kona Sutra plus few modifications would be my choice.
Let us look at the specifications of those bikes and what problems they have.
A common weakness is that none of these bikes is sold with a front hub dynamo, so you can't have reasonable lighting solution without changing the front hub or the entire front wheel. Another common weakness is that all those bikes on the list have a triple crankset with a ridiculously small "granny" ring that's so small not even grannies would use it. Also as is typical of most bikes sold today, the spokes are not optimal: for today's hubs, 2.34mm/1.8mm/2.0mm triple butted is optimal but bikes are today usually sold with straight gauge or double butted spokes.
In addition to the common weakness, you are paying a lot of money for a Brooks leather saddle that takes huge efforts to maintain, takes about forever to break in, and will be destroyed in the first rain.
The wheels in Sutra require an immediate change too: the 622-23C rims won't take reasonably sized tires (so the stock tires are 622-40mm, too wide) and the hubs are cheap no name Formula hubs -- but you would be changing the front hub anyway as it has no dynamo.
As good features in Sutra, it has thru axle (which is necessary for disc brakes to not be fatally dangerous), the frame is butted chromoly, the bottom bracket is apparently Hollowtech II, the bottom bracket drop of 72mm is about optimal if you choose a size with 170mm cranks (for optimal BB drop, calculate 240mm minus crank length for conventionally sized road tires), and there are disc brakes (although the brakes are mechanical not hydraulic -- good for maintaining, not so good for braking feel). The shifters, bar end shifters, are very reliable, not likely to break in a crash and can be operated with even thick gloves.
Trek 520 disc has more problems:
The frame has disc brakes and quick release wheels, a no-no. This is fatally dangerous. The front disc brake causes the front hub end to move slightly during braking, wearing away the metal, loosening the quick release, until the quick release is so loose the wheel pops out of the dropouts when braking hard.
The fork seems to be aluminum not chromoly. Fork is not the place to experiment with "novel" materials. If you are a fan of aluminum, choose aluminum seatpost, stem, cranks etc. And if you really like lots of aluminum, then perhaps the frame could be aluminum. But fork? No!
The rims appear to be 622-21C so you can't fit narrow high performance tires. The shifters, STI Sora, cannot be operated with gloves on, cost a lot of money, and are likely to break in the first crash plus are likely to wear away.
As good points in Trek 520, it has Shimano Hollowtech II bottom bracket, Shimano hubs (but you would be changing the front hub anyway as it has no dynamo), the frame is butted chromoly (although the fork is not), there are mechanical disk brakes (easier to maintain than hydraulic but poorer brake feel). The bottom bracket drop is 75mm or 70mm depending on the frame size, not totally unreasonable.
The weight limit of Trek 520 is stated as 125kg, leaving only 110kg for the cyclist and the cargo. A bit heavier cyclist and you're over the limit with no cargo. However, it could be the lawyers just wanted to play safe and put a low weight limit on it.
Surly Disc Trucker has problems too:
The bottom bracket drop, 80mm, is way too much for 170mm cranks using conventional road tires. You will be hitting the pedals to the ground in corners. Not only that, but attach 175mm cranks and the bike is nearly unridable due to its low bottom bracket. The handlebar is a strange riser drop bar which means if you need a new handlebar you can't find an identical one and a non-riser drop bar changes the geometry of the bike. The hubs are cheap Novatec ones (but you would be changing the front hub anyway as it has no dynamo). There appears to be no kickstand mount. The shifters are STI Sora with their already explained problems. The tires are ridiculously sized 41mm. The frame is unavailable with 622mm conventionally sized wheels in the smallest frame sizes.
As positive points in Disc Trucker, it has thru axles (good for disc brakes), frame and fork are butted chromoly, there are mechanical disc brakes (tradeoff vs hydraulic), and the bottom bracket is Hollowtech II.
My opinion is: get the Kona Sutra as it has the best frame+fork combination (for example the BB drop of Disc Trucker makes it unusable and the aluminum fork of 520 is bad experimentation). You will want to change the wheels anyway to get front hub dynamo, Shimano hubs, triple-butted spokes and 622-19C rims with double eyelets. The tires of Sutra need changing too to something like Grand Prix 5000 in 32mm size, and unless you're a masochist the Brooks leather saddle needs to be thrown away. If you find the non-optimal chainline of the triple crankset a problem, you can easily change it to a double one, the bar-end shifter makes it easier.