You might want to give more info about what you are looking for, where you race, and how big you are. Understand, no kart will make you WIN, you win a race in the shop before you show up in prepping tires and preparing your kart. If you are a small guy you could run a 115cc or a set of dual yamaha's are very fast and versatail. If you are a big guy you should look at a like a MX engine, the 250cc 2 strokes that Tim Taft is building the bottom ends for are very lite and very powerful. The 450's make big torque but are a little heavy and be a little rougher to setup depending on your kart and body weight. If you are a medium size guy you might want to look at a stroked Sudam or a BRC150, but we all need to know more info to steer you in the right direction.
You are a heavy guy. You need HP and torque. Go buy Justin Westerfield's kart if it is still for sale. It's not cheap BUT it wins and it is a quality piece. You would be wasting your time on about anything else in the UAS. If that doesn't work out, call Jerry Stevens. That will be a JAWA. That's even more HP and more torque BUT expect to pay more money. Those are both top tier karts. It takes that to win.
I know where I can find you a complete, ready to race 250 2cycle shifter kart for $1500 but you're not going to win in your area with it, unless it is a heat race and you get super lucky on tires. Being your weight puts you in a small box of options. I'm 6'1" but you give me 60lbs and I have 18 years of racing experience. I can get by with an open Sudam. You're going to be over 430 lbs with a kart. You need all the umph you can get.