Seller listed that the posts are included, but that doesn't necessarily mean they're functional. Obviously can't test without lids.
It's pretty rare that posts get damaged enough to need replacement, so you likely won't need to replace any of them. It's more of a concern if they're missing. Same with the diptubes.
Are you sure this is accurate? I have some firestone cornelius kegs and I can switch out ball lock or pin lock posts without any problem. If I can do that I don't know why those post wouldn't work on regular ball lock kegs. If he doesn't need the posts he can buy universal poppets and o-rings for cheaper anyway.
I do agree that the total refurb cost depends on what else may be missing.
As I said, different keg manufacturers and models have different post thread sizes. The fittings in the kit you linked are both threaded for 9/16"x18, which only fit pin lock kegs made by Firestone and John Wood, or the gas side of newer Firestone ball locks. If you have pin lock and ball lock kegs that you can interchange all of the post fittings on, then they're Cornelius R pin locks and Cornelius spartan or super champion ball locks, which all have 19/32"x18 threads. The kit you linked wouldn't fit those.
FWIW here's the thread sizing for various keg models-
Firestone V Challenger, Firestone VI Challenger, Firestone Super Challenger
Gas 9/16"-18
Liquid 5/8" -18
Cornelius Spartan & Super Champion
Gas 19/32" - 18
Liquid 19/32" - 18
Cornelius R (pin lock)
Gas 19/32" - 18 (2-pin)
Liquid 19/32" - 18 (3-pin)
John Wood 85, Firestone Challenger
Gas 11/16" - 18
Liquid 3/4" - 18
Firestone A, Firestone R, John Wood RA, John Wood RC (pin lock)
Gas 9/16" - 18 (2-pin)
Liquid 9/16" - 18 (3-pin)