I have a '98 C43 AMG and it is an exceptional car for almost any application; I use mine for mostly for daily commuting (alone, without passengers) on US interstate motorways and urban streets -- great combination of power, handling and maneuverability. The C cabin is tight, esp in the back seat, and the luggage space in the boot is OK, but not like a S class (see below).
That said, if your friend wants the car for long highway trips, it may well not be the most comfortable choice as the ride is very hard (the C43 is probably the "rawest" of the AMG cars and the C36 very similar)-- with Michelin Pilot Sports, I get a jarring ride on poor pavement, "tramlining" on uneven pavement (particuularly bad where lanes have been moved for construction and you have to drive on a road-edge seam) and a "pop,pop,pop" on highway section seams or cracks. The wife and kids think the car is too bouncy and jerky for comfort (the poor ride quality is much more noticeable for passengers than for the driver, who is glued into the great road experience).
I think it's just the price of the AMG suspension and performance, but a number of drivers have switched to better riding cars after finding the C43 a bit hard-edge for their tastes. If your friend wants a comfortable long-legged highway cruiser and doesn't really need razor handling, he might instead consider a W126 (say a 420SEL from the 86-91 era) or a W140 (S class from 1992) -- an older W140 will be about the same price as a newer used C43 or a bit cheaper, and the W126 will be much cheaper. Either S class car will have much more luggage and interior room and be far more accomodating for passengers, esp more than two, as well as having more mass (safer) and still able to handle sustained autobahn cruising.
The fuel economy on the C43 is OK (it takes only premium), but the AMG parts are extremely expensive -- e.g., brake rotors alone are $800 for a pair (parts only, not counting labor) and usually have to be ordered by the dealers since the special AMG parts are not generally stocked due to the small number of AMG cars.
In summary, your friend might want to do it backwards -- get the C43 for individual daily use and get an S-class for long highway trips -- that's what I did.