Only you can determine what price is right for you. I would consider that the kit is revamped so the "historical" or vintage value is decreased. Also this may be a pieced together kit, which plays towards the previous statement.
It will come down to the simple fact of the shells really being of Slingerland origin, the current condition of the shells, and the sound quality of the kit.
Is this kit local to you? If it's not you will never know what that quality is prior to purchasing (this is the typical online purchase gamble as even sound clips can be of dubious origin).
If it sounds great but the kit is actually a set of tweaked CB700 or Maxwin shells would the purchase offer value? Would you feel taken advantage of...
That's what should shape your offer price. If he can't substantiate or prove they are Slingerland shells (or that Buddy Rich f a rted on his throne, or John Bonham spilled beer on the floor tom, or whatever happens to be in a given ad) you should only offer what you think is reasonable for the risk of investing in the kit as presented.
Minimally ask for high resolution pics of the shell interiors to help evaluate the shell condition. Also consider tallying up the value of all the parts to come to some valuation of the "whole" deal.