I have never been a huge fan of 404s, even empty they have fairly low performance but if you already have one why not use it?
They are one of the fastest original type mogs but they will slow down on hills because the gaas engine is gutless. You'll be doing 55 max on the flats without a load and people say you should stop every couple of hours to let the tranny cool because it uses journals instead of bearings. There are a few things to make it faster but if you are going to do that you may as well get a faster mog to start with (yes, this will be diesel).
They are rock solid and fairly difficult to break, you probably want to take the mil spec ignition off and replace it with an electronic ignition and civilian wires/plugs.
As for parts they are easy to get NOS or from donor trucks and they are typically cheap but a lot of the bigger stuff is no longer available from MB.
You don't inherit the world from your parents, you borrow it from your children.
1979 Unimog 416 Expedition Camper
1974 Unimog 421
2004 Dodge Ram 2500, 4x4, Double Cab, Cummins Turbo Diesel
2006 25' Airstream International CCD
2009 Harley Davidson
Sugarloaf, Boulder, CO