Unless you are racing, I would look into a 2004 KX 100 or a late 2003 left over RM 100 since there are no more new 04 RM 100s.
But for 85s, basically: YZs and CRs are the top end monsters, and the KX and RMs are the low end monsters. If you are going to be doing pretty much primarilly track riding, the YZ or CR is for you, but if you are going to be riding at other places than tracks, like opened feilds, occasional trails, etc, a KX or RM is for you, since the powervalves will also come in handy. But IMO, your best bet is a KX or RM 100.