I agree with everything
@DanR said.
You may want to consider this though. If a used amp is $4000 and might need some work, you might want to look at some demo amps which are sold as new and have a warranty.
I know of a local dealer who routinely offers demo amps for steep discounts. Some amps that came up recently included a Krell DUO 300 XD Class A Amp, which is the model I happen to have and is great, and I think the price came down to $7250 when it was sold. So this was less than half the retail price.
You may consider contacting Shawn Millin of DaVinci Media who advertises here at MLO. He has demo units for sale from time to time also.
My point being that if the cost of servicing is $1000 to $2000 for an amp that has no warranty, then paying just a bit more would get you a warranty and an "almost new" amp with very few hours on it.