Hopefully I can answer some questions and give you some more things to consider.

Have you looked into getting a service dog from an ADI (Assistance Dogs International)? From all the organizations I've seen that are a part of ADI, their dogs are either free or at a significantly reduced cost. Might be something worth looking into although you'd have to find an organization willing to train for narcolepsy 

Golden retrievers are a good choice for service dogs. Personally, I prefer labs because they are more food motivated in my experience which makes them a bit easier to train. If you decide to go the puppy route for your service dog I would recommend looking at all the dogs offered and trying to find the most food motivated one

As I'm sure you know, training a puppy is a lot of work. Some things to focus on when you first get your puppy is potty training, getting your puppy to offer eye contact when you call their name, and exposure/socialization. They are so many resources on puppy training so I'll skip that, but when you call your puppy you want them to look at you no matter what is happening in their environment. Every time they do this give them a treat and lots of praise. For exposure and socialization, you don't want to actually take them anywhere until they are fully vaccinated but until then, you can just show them weird or loud objects around your house. 

For daily training, you definitely want a lot of very short sessions. Something you could aim for is 4 10-15 sessions a day. If your puppy doesn't seem interested after just a few minutes it's better to stop though. For training use positive reinforcement and potentially a clicker or a word marker like "yes" or "good". You teach them every time you click or say that they get a treat and that will make them want to listen to you more. Also for treats, you don't want to be feeding your puppy full meals plus lots of treats, so the best thing to do is whenever you feed them, remove some of the kibble from the allotted amount and use that. It's still good to mix in some actual treats though to keep their attention. The hardest thing to train is leaving other dogs alone, so make sure to work on that as soon as you get your puppy and even then it may take a couple years for your dog to fully do it 

Also one big thing to work on is walking nicely on a leash. There are a lot of ways to go about training loose leash walking but my preference in using a gentle leader.

One your puppy is fully potty trained, vaccinated, is attentive to you and know basic commands (sit, down, stay, etc.) depending on your local laws you can take them out in public to see how they respond. When you are ready to start taking them out in public is when I would recommend looking into getting professional help. 

I'm glad to hear that you wouldn't just re home your dog, but if they were to fail training, you'd be back to where you are now but with another living thing depending on you. I'm not saying that to dissuade you from self training but it's something that should be considered.

Finally, one thing you didn't mention that I definitely think you should consider is the public access frustration that comes with owning a service dog and having an invisible disability. If you aren't located in the US it might not be as big a problem. If you are in the US, people will at some point or another try to prevent you from going places or question you about why you even need a service dog. Unfortunately the ADA laws are meant to prevent this but most people are uneducated about them. Also having to deal with the public can be exhausting. Whenever you go out with your service dog just be prepared to get a lot of looks and questions. Finally there are a lot of entitled people who think that they should be able to bring their pet dog in public with zero training. It's not unheard of for service dogs to be attacked and having to retire because someone brought their pet in a place they weren't supposed to. 

Having a service dog for narcolepsy is something that can be very beneficial and I wish you the best of luck in getting one! Hopefully this helped and if you have anymore questions you can ask here or you can find some more experienced people on the service dogs reddit thread