I have a shifter kart, Birel Chassis, Pavesi engine/trans, before that I had a YZ 125 ....
a link to a forum http://www.ekartingnews.com/forum.php
A 125 on a good size track will do 120MPH (good racing tuned engine will be @ 45-49 hp), the kart (with out driver) will be 180-200 lbs
They are very fast, have raced since 1991 some sort of kart (did not race last year).
It is not very cheap .... that would be my #1 complaint... cost.
It takes a bit of space to run a shifter kart, if you build one think about a place to use it.
A racing frame will have about 1 1/2" ground clearence max. and the seat could be 3/4".
Think about that compared to what surface your will be driving on. It might be better to build something.
Here is a funny one ... (to name a couple) Danica & Sam Hornish both raced with our tour ... I missed Tony Stuwart by a couple years.