I am a big guy just like you. I love 27" frames and I ride them like they are. They are too expensive to upgrade to modern equipment. Go spend some money on new brake pads, new tires, new tubes, new cables, new handlebar tape, and clean the bike up. Ride that vintage Continental like you are the boss of the road. Get into shape and in the meantime start researching bikes and keep checking out craigslist. The most important thing is to get out and ride. You can get some riding shorts, a jersey, helmet and all the works too. Spend your money on equipment to get you out riding and get your Continental in prime working condition. You should easily be under $300 bucks to get you and the bike out on the road. That is what I would do.