Christian,
Nice winter bike build up. There are a couple of companies that make a long reach brake for 700c conversions. I think Tecktro has the largest selection and best cost/performance ratio. The problem with the old road bike brakes is that the need a little different mechanical advantage ratio on the brake levers than the new brakes use. Since you are using the new brifters, and you have done such a good job on building up the bike, I would suggest new brakes.
One good place with a good selection is Universal Cycles. There are two schools of thought on winter bike brakes. One is to have really good brakes so you can stop fast if necessary. Or brake hard on dry patches so you can release on slick areas. The other is to have brakes that don't stop very hard so you don't accidentally cause your tires to skid on a slippery surface. I myself prefer to have good stopping brakes on the winter bike since patchy slick conditions are common for me.
In regards to the chain. Most of the new 20 dollar chains are made out of alloys that while not as rust resistant as stainless steel have good enough corrosion resistance to last a winter season if you keep them clean and lubed. No need to go stainless steel. The cheap 8 dollar chains are more of a problem and I would avoid them if possible.
I've never used leather bar tape so I can't comment. But I think it would be advisable to put something on them. Bear in mind that when you are sweating chemicals can leech into your hands. So I would use something safe and natural like Snowseal. Which is Bee's wax and Lanolin I believe.