I'm with the vintage guys and say not to upgrade this bike.

Now that said, why upgrade the brake levers, if the old ones work don't replace. with an Alfine 11, it comes with a rapid-fire shifter. I'm not sure if the alpine shifter will fit on the wing handlebars. But if you did find some brifters that work with the alfine for $250 that cost is in the ball park of brifters on the market.

You would also need to spread the frame to get the gear hub to fit.

After looking at your bike a little more, I would just suggest that you get a new bike with the equipment that you want on it. I just think that the cost is too much for upgrade.

I am not sure if you will have some of the same problems that I had trying to mix older equipment with new components. I was trying to use a older dura ace cranks with a 10 speed campy drive train and the 10 speed chain kept sticking to the cranks. And it appeared that the teeth were to wide for the chain.

So I am thinking that you will need to replace the brake levers, rear derailleur, hub cranks, stem and handle bars, chain, cassette. Well for few of hundred dollars more you can get a new bike, and you won't have to worry about compatibility issues, respacing the rear triangle and chain line.

Also now you will have a second bike to use. N+1 is good (to a limit).