Memphis does not make a '5k'...
The Mojo4k (specifically the 16-MC1.4000) is rated at 4000x1 at 1-ohm.
These can be modded to to do about 4500 watts, but of course will void the factory warranty.
A fair heads-up on this particular amp though, the failure rate is pretty high, it's rare to see one of these re-sold (used) that hasn't been rebuilt by a 3rd party.
The Mojo 2500 is far more reliable, honestly I'd do a pair of those if your needing a good amount of power - vs. the 4k...
As far as price, is he selling it as 'new' and with warranty? Retail price I believe is $1200ish, but not sure what kind of deal he's planning to cut with you.
Used, they go for $500-$600ish depending on condition and year.