If you have your 97-04 KX500 suspension set up correctly then I think you will be happy.
My 97-04 suspension will allow me to go over whoop sections far faster then I really want to go.
Everyone that it around my weigh that has rode my bike all say that the suspension is really good.
That being said, I still need to do a conversion on mine to see if it makes any difference for me.
There are a few on this site that have done the conversion on the front end.
I hope they chime in for you.
As to who to use... that's like asking which oil to run I currently have full Race Tech gear in mine.
I have also ran Enzo, Factory Connection, House of Horsepower, and one other that I can't remember the name on.
For me Race Tech is the best followed in this order..
Enzo
Factory Connection
Others, whatever...
Hopefully a few other guys will chime in..
Warning regarding Race Tech..... When you go through their interview process.... Be honest with yourself about everything...
If you tell them you are faster then you really are, chances are you will get a suspension that is too stiff.
Also, you have to understand what they mean when they say "Skill Level".
Dirt Intermediate/Expert/A Class is not where you are at unless you can qualify for a National.
That is what the guys at Racetech told me direct.
Most of us older guys will be Dirt Novice/ Intermediate/ C Class..
Mine is currently at Dirt Intermediate/ B Class.
For me it is perfect as I have come close but never bottomed it since the rebuild.