I was tossing up between the 400 and 650 too.
In the end I went for the 650.
Simple mechanically, plenty of grunt, air cooled, plenty of places to attach a luggage rack and plenty of room to work on the bike.
My only problem is the small tank, but I just carry a $5 jerry can. The advantage of a jerry can over a bigger fuel tank is that you can jettison the jerry can to reduce weight in the more populated areas, and then buy another jerry can when you head out into the unpopulated areas. You're not stuck with a 40 l tank everywhere you go... It's also way cheaper. IMS tanks etc are quite pricey and not really needed in my opinion.
I wouldn't take a fuel injected bike RTW. If an injector clogs up or shits itself etc you can get into all sorts of trouble. The DR650 carb takes about 5 minutes to take off, disassemble and blow out using a small air hose and the air in your tyres. (Carry a small pump to pump your tyres back up when you're done)
The 650 is also quite a bit cheaper than the 400.
I'm 6'2", 92 kg. I don't have any problems pulling an 8 hour day on the 650. Put a bit of sheepskin on the seat if you want a bit more comfort. I'm in the process of making a wind screen out of a Lexan off-cut and a luggage rack for it. I'll post some pics when I'm done.
I can't advise on parts because I'm not sure exactly where you're going... Have a look at the global Suzuki site for dealers around the world.
Brake parts, some spark plugs and a few brake/clutch levers is all you're likely to need. Consider taking a chain and some spokes too if you're going to be miles from anywhere for more than 10,000 km. I know a guy who did 31,000 km on his DR650 through asia and the mid east and it never missed a beat.