Like others have said, you could get a decent aluminum bike with 105's on it. Unless you have knowledge about bikes, I wouldn't recommend buying used, unless it's from your trustworthy LBS. And buying new from an internet retailer is probably not a great idea for someone with limited knowledge.

As for your fitness, losing some weight off your small frame would definitely help your running and riding. Work on cutting calories while continue to train. A reasonable daily deficit would be in the 300-500kcal/day. You can use myfitnesspal.com to track your food intake.

Good luck.