If this is the bike to last you for a long time. Use your 1500 and get yourself a custom frame. Parts aren't that big of deal, stuff goes into fashion nowadays just as quickly as it fades. And there are a ton of custom options out there. Hell, a custom Kalavinka is under 1500 bucks. If you don't feel that's an option, then get a road bike because a 1500 dollar road bike would be faster than a 5000 dollar fixed gear.