This bike has never had disc brakes but I'd like to add some to the rear wheel. I do plan to keep the front system as is. What am I getting into or better yet where do I start?

Why do you want/need disc brakes? I have discs on my Volagi and really like them for hard descents, but depending on your situation you can certainly achieve very good braking with calipers or v-brakes. Discs don't seem like a very practical add-on- I'm just not picturing how you attach the caliper and you would need to rebuild your wheel with a disc brake hub.

I figured that was the mounting bracket for a disc break system. I wasn't sure if it was going to involve rebuilding. I've seen the universal disc brake kits but again no experience with them. I'm doing this for a cleaner look so I can remove the cable to the rear of the bike. After adding a rack the cantilever brakes are in the way for adding fenders.

At the minimum you'll need the disc brake kit and a new rear wheel. You may have some fun aligning the caliper to the disc brake rotor.

But what are you trying to accomplish? Typically diamond frame bikes already have more rear brake than they can use. If I were to install only one disc brake, I think that I'd want it on the front.

That's why I come to these boards for good advice This is purely aesthetic and trial and error basically to see if its possible. This is my crap bike I've had for 10 years and its been through many phases (currently a cruiser). Plus like I said I can't get rear fenders on it. This bike is going with me to Orange Beach at the end of the month. My other bike is an Electra Cruiser.

As the whole setup includes a wheel + all the disc parts, consider selling what you have
and getting a new bike already built with Disc Brakes.,. actually a Savings,
and the rest of the bike is Fresh & New too..

You'll get more out of a disc on the front than you will on the rear.

Adding a disc brake to the frame won't be a prob,but you will need to either replace the rear wheel or rebuild it with a disc hub. You'll also need to replace that rack. If you're running canti's,you'll need to make sure you get a 'road' disc caliper to match the current short pull levers.

If the problem is the current brake getting in the way of fenders,then a much easier and cheaper solution is to just swap the brake. Canti's come in different styles with arms that go from almost vertical to completely horizontal. Any shop or even co-op should be able to set you up with what you need. Also,how is the brake interfering with the fender? Are the arms or the wire hitting? It's possible that the straddle wire just isn't set up right.

Post a pic of the brake with a fender.

As you can see there isn't much room for a fender.

Smaller tire would make room for a fender and those are not canti-lever brakes, they are v-brakes.

+1

My bad lol

Maybe I'm missing something, but a disc brake will still require a cable. It just doesn't go to/end at the same place on the frame.

Disc brakes make mounting a rack close to impossible. I converted from V brake to disc. Discs look way cooler and have the advantage that a less than true rim is no problem. For a street bike, that's the only real pluses.

With careful trimming of the fender, the V's will work fine.

Guess I could go coaster brake and spend a bunch of money.

Only if you also mean changing to an internally geared hub or single speed plus chain tensioner.
Otherwise no.
Edit: That may not have been clear enough.
No. Just no.

Chain tensioner is already there. Guess ill work on rigging a fender on there.

Not with disc specific racks. Also,many bikes mount their calipers on the chainstay,which allows you to run any rack.

They also work way better in snow,require fewer adjustments,are easier to adjust,pads last longer,and they don't cause rim wear.

Another +1.

From that pic,your tires are too wide. Go with a slightly narrower tire and fenders will fit.

Discs (of midrange cable style) are useful in wet, slushy, muddy conditions but in the dry offer no braking advantage over V brakes. You will need a new wheel, brake caliper and cable. The existing controls should work OK. Under hard braking, the rear does very little work, the weight is moved to the front wheel and this does almost all the stopping.
Disc-compatable racks have a large spacer which moves the struts 1" wider each side. The luggage is 2" wider which increases air resistance and reduces the ability to squeeze through narrow gaps.
Adding up the pros and cons, there are very few pros.

Many of these are advantages only for high mileage riders. I never noticed significant wet weather performance between V and disc brakes. No snow experience myself. I intend to put a front disc on my all arounder, mostly because I have an extra one. Discs still limits your choice in racks. I like discs. I still believe they are oversold.

Keep the V-brakes. Get Kool-Stop "Salmon" brake pads, and your brakes properly adjusted. You will be stopping quick enough. I used to mountain bike, rims and brakes covered in mud, never an issue stopping (way before I finally upgraded to Kool-stop...last year).

If your going to put ONE disc-brake on a bike, put it on the front. But I don't see a need for them for 99% of riders. We never used to have them, and people stopped just fine.

I converted this bike to single speed and if I could just go coaster with only replacing the rear wheel I would. Yesterday I was in the garage and found the fenders off my folding bike which I may be able to rig.

No worries! I have four bikes with disc brakes that I have put rear racks on all of them myself. If you live in a rainy place like I do here in the NW disc are the best way to go. I wore out a set of rims on my then new Trek 520 in less then one winter. I have only a twenty mile a day commute. I used the salmon brake pads which is the best pads to use here. I also made it a point to wash the rims every time I got home after ridding it in the rain. What more can you do? To this day I have yet to replace one set on rims on any of my disc brake bikes. Disc just stop better in all weather and don't wipe out your rims! Just my $.02

Just buy a used MTB with Avid BB5 or BB7 brakes.

The better rear disc caliper mount is inside the rear triangle (IMO) because it will not get in the way of fender/rack mounts and (IMO) they are less apt to grumble/howl like calipers mounted only to a seat stay.

Discs are so much better for mud, grit, rain, puddles and controlled panic stops.
My Trek DS has hyd. discs and it will be hard to go back to a rim brake except on a competition bike.

There are several inexpensive disc bikes at BD and by the end of Summer they might have hyd. disc bikes.

I've flipped over my mountain bike with v-brakes and disc breaks all the same lol