I don't know if disc brakes are a must, I think front and rear brakes are a must. But, disc brakes (when properly tuned) are great.
I have a similar bike I'm about to build into a shifty. I can't wait, either. I've spent probably 30-50 hours on this site just using the search option, and looking at what people have done. I've learned a lot on what to do and what to do, what to order, and what to stay away from.
If you do the same, the install should not be a problem. I went to the walmart close to my house and measured about 20 different bikes. I made sure the seat post was over 1.5 inches away from the rear tire, and even made sure the bike has enough clearance for the engine with the shift kit installed. I figured, I'd measure 20 bikes, and just order one shift kit, lol.
So, I'm ordering the shift kit on Thursday, and I'm going to pull the motor off my old bike, because it just looks too cool now, I just want to make it a pedal bike only (since I put a banana seat and ape hangers on it, lol. It looks like a large version of a 10 year old kids bike, lol.. So, like I said, I'm going to be doing the same thing you're doing on yours. Keep checking this site, I'll update with my build. And when I do that, hopefully we can exchange techniques and "oh, by the ways".
So, best of luck to you. Did you order your stuff yet?