What do I need to know?

Bumping this one up!

Disks are nice and will bite well, but are not quite an mount-and-go prospect. The thing about disks is that you need a disk brake hub to attach the disk to, as well as a place to attach the brake caliper. The stock front fork leg will not have the lugs for a disk brake caliper, so you will want a new fork or have someone weld on mounting lugs. If you do not already have the wheels you are going to use, be sure to look for a wheel that can take a disk. And 12 gauge spokes are nice too.

The easy route since you already have a front mounting hole (for a front fender) is to get wheels that work well with side pull caliper brakes, and install those. You probably have a fender mounting hole on the back that would work as well. From what I have seen, there are decent quality side pulls for pretty cheap on Amazon, and you can install them yourself no problem.

If you wanted to go with a springer front fork, they have springer fronts that have the lugs for a v-brake already installed, which will give you more bite than a side pull caliper. And I think Venice makes a disk brake adapter for springer forks?

Personally, I am going with a Sturmey-Archer 90mm drum on my front, the XL-FD. They have them for about $70 on Amazon, and my bike shop said they could lace them into my current 12 gauge spoked wheel for pretty cheap. Those stop well, look old school, and are not bothered by the rain. If you are not running a hub adapter on the rear for your sprocket, you can get a 90mm drum for the back too.

I hear really good things about the Sturmey-Archer 70mm and 90mm drum brakes. It replaces the hub (center part) on your wheel, so no modification to your fenders are needed, BUT they do need to be laced into a wheel if you do not buy a wheel with a drum brake already installed.

As far as the speeds, if you are not going to use a shift kit which ties the motor into the bike's gears, they are still handy to have since there is at least a little pedaling involved to get moving. And if your motor dies when you are away from home, you will be very glad to have those gears! The bad news is that I do not think anyone makes a hub adapter for the drive sprocket that fits a 7 speed hub, so you would either have to use the kit supplied rag joint (which a lot of people use, I just did not have the patience to get it set up right) or go with something like a top hat adapter. No biggee.