You will certainly pay a premium purchasing via AudioClassics; however, I've heard nothing but good things about them. They are considered the premier dealer of vintage McIntosh gear and I've never heard of someone having a bad experience.
If you're willing to take a little more risk, the regular used gear sales can come cheaper. Just be sure the seller knows how to properly ship the item! When I purchased my MC452 from a private seller, we agreed to have LMC, another well-known authorized McIntosh dealer, handle the shipping so that it would be done correctly. These amps are not light-weight and replacing shattered glass can be quite expensive.