As you probably already know, the xc-w is going to give you the wide ratio transmission. In 2006 the 300 underwent a pretty major enging change and it has stayed pretty much the same since then. In 2008 they added e-start to the xc-w and to the xc in 2009. The starters on these bikes have had some problems and it would not be a surprise to find one that is in good shape with a non working e-start. Spend a little time on ktmtalk and you will find that there are solutions to this problem.
Some questions I typically ask are:
1. Have you put a new top end in the bike?
2. How often do you change the transmission oil?
3. Do you clean the filter after every ride?
4. parts just wear out on bikes, what are some of the parts you have replaced on this bike?
5. How long have you owned it?
6. If they are not the original owner I will ask what they know about the how the previous owner took care of it.
Don't be afraid of a bike that has been raced. I have found that most bikes that have been raced are better maintained then bikes that are owned by your typical weekend warrior. Use your instincts on the matter and if you don't feel the seller is being honest, pass on the bike.
I found this one on KSL. I don't personally know the guy that owns it but I do know some of the people he associates with in St. George. I would not hesitate to look at it. His asking price is not bad, but I think you could get it for a few hundred less than what he is asking.
http://www.ksl.com/index.php?nid=218&ad=28076711&cat=236&lpid=1&search=ktm 300&ad_cid=1