having owned millions of them i would suggest depending on your size a uk tzr125 is a cheap type, not teh racer style most mention but ist cheap as fuck to fix get bits for and taking them from 12bhp/85mph std to c.25 and 110mph is relitively simple
ive had import tzr's and found them constantly troublesome, 1 has only 2k and was alot of hassle, i still have an aprillia 125 in my shed, its teh fastest they did before around 2003 (when i stopped caring) and its horrible to ride
in teh big picture i woudl say do you cbt, then do you test on a 600 (summet shit the test place give you) then get anything you fancy
i passed on a shitty 600 bought a fireblade and thats what i ride now, no need to upgrade later except to newer and it goes as fast and you mit makes it, infinatly reliable, doesnt annoy neighbours like a 2 stroke etc etc