GS and an SV are excellent suggestions for first track bikes. They said everything else, so that'll be my contribution....

 

The GS 500 also has its own site "GSTwin". Its a awesome little site, hasnt been updated for a while but great info.

 

If you want a dedicated track bike then get something already prepped. You'll save plenty of coin as the previous owner has already taken the hit. If it's recent year, you might even sell close to your investment if you don't keep it too long.