1. Chainguards that run along the top length of the chain have always been sufficient for me. I'd start riding with the top chainguard and then shop for a full-coverage guard if you find the original isn't doing the job.
2. I find the learning curve to be pretty short on most shift systems. Once you get it down (usually less than a day for me) it becomes pretty hard-wired, and and you should be able to hop between the different bikes without issue (sort of like switching between manual and automatic transmissions in cars). I also think being able to shift while stopped is a big advantage, as is the low-maintenance nature of internal gear hubs (IGH). Another advantage is the IGH always maintains a straight chainline (the angle of the chain relative to the frame.) With IGH, the chain runs parallel to the frame, nice and straight from the front chainring to the rear cog so it pedals very smoothly. On derailleur systems, the chain can run diagonally front to back when certain gears are engaged. This puts more friction on the chain which makes for less smooth pedaling and faster wear. The chain on an IGH is also tighter, so you don't get that chain noise (or chain slap) over big bumps as with the relatively loose chain on a derailleur system. A disadvantage with the IGH is they can be heavy, but that's not much of an issue on a load-hauling city bike. They are also kind of pricey compared to derailleur systems, but that decision is up to the individual. As to how many speeds, a 3-speed should be fine for normal riding, but you might want to step it up to more gears since you are planning on load hauling.
3. A link to the front rack you are referring to would be helpful. Rack selection depends on more than size and load capacity. It also depends on which bike you get and what sort of braze-ons it has (mounting points on the frame/fork).
4. Haven't ridden any of these myself, so can't speak too much about how good they are.
Last edited by Brennan; 09-17-14 at 07:00 PM.