Re: disc brake conversion
I don't know how much you know about bikes so I am going to go into detail.
You will need a fork that has a mount for a caliper. This I am 99% sure will only be a mountain bike fork. They do make them in a solid (non suspension) mountain bike fork if you want to keep the cruiser look.
A bike fork has a part called a steerer tube. It is the tube that goes through the bike frame. The hardware that has the bearings and holds the steerer tube into the frame so it can rotate is called the headset.
You will most likely have to change the headset because cruisers use what is called a threaded headset. This means the steerer tube on the fork is threaded. Mountain bikes use non threaded headsets. Some older mountain bikes use threaded like your cruisers but they won't have the tab on the fork for the caliper.
Since you will have to change the headset you will need a new stem (the part that attaches to the handle bars.
You will also need a new hub that has mounts for a disc for the disc brake. This means you will have to take your wheel apart and have your painted rim laced with new spokes to your new hub.
As a recap you will need a new hub, spokes, rebuild wheel, new fork, headset, new stem at the least plus the brake and brake lever.
Here is the break down in cost:
brake - 45$
lever - 20$
headset - 30$
spokes - 35$
shop to build wheels -35$
That is 280 -330$
You could do it cheaper with used parts on ebay.
Another thing I need to mention is they make different diameters of steerer tubes. The two most common is 1" and 1 1/8". Mountain bike forks are most commonly 1 1/8". I am not sure about cruisers. The bike shop can tell you what size it is. If it is 1" it will be hard to find a 1" with disc brake tab fork but probably not imposible.
Hope this helps