coming form experience don't go for anything that has been put together form two bikes or/and you don't know the history of the bike like the mileage etc. also a bike that has been used for racing has most likely been striped and re-built a hundred and one times and if the person is a half a job harry the bike is going to have a few bolts missing here and there, also if the bike is made from two separate bikes it would suggest that there was a f**k up along the way so they needed a cheap replacement. i had problem with a bike that was made from two different bikes and all was good till i found out that the bike's engine number was different to the frame number so who ever put it together didn't let any one know (DVLA) about it for some reason :P but any how go for a reliable bike witch you can find out the history of etc..
hope this helps
