steel is definitely the best choice - it's stronger, more easily fixed (welded or bent) and is easier to get a frame adapted to your needs. make sure the bike handles very very well when fully loaded before you go - a fully loaded bike that handles badly can be a recipe for disaster.
make sure you have good eyelets for your racks - get some brazed on if the frame hasn't got any (get the type that have a lot of screw thread, and use long bolts to hold racks on). get some extra bottle cage bosses under the DT at the same time.
get decent quality racks (I can highly recommend tubus steel racks)
as for equipment, well cup and cone hubs are far easier to service than sealed bearings (and good quality ones will last a long time before they need servicing anyway). an 8 speed freehub body is a good idea, you can always use a 7 speed cassette but you'll still have all the options, just in case.
avoid stuff like hydraulic brakes - they may work for years without a problem, but what will you do if they do break down miles from anywhere?
make sure you have plenty of hand positions - either drops (maybe with tri bars or similar) or flat bars with barends, etc.
steel chainrings are a good idea - they last a lot longer than alloy ones
tandem hubs might be nice but you might have a problem fitting them into a normal frame. go for 36 hole, with decent spokes and (very) strong rims. good tyres are a must too. and a good patch kit (for tyres and tubes)
and look after the little things too - a pump that breaks is no use at all, and even less if you're 50 miles from anywhere or anyone with a flat tyre.
and don't worry about weight - there is absolutely no point at all in spending extra money to get lighter parts, especially if they're not as reliable as heavier/cheaper ones. the number one priority for a bike like this must be reliability, followed very closely by serviceability. when I was in australia, a quick calculation of the weight of my fully loaded bike was 60kg at it's heaviest (going down the middle, when 15 of those kg were water). even without the need to carry so much water, it was still approaching 50kg. so saving a few kg here and there makes no difference at all really!
I'm sure there are lots more things I've forgotten - it's been 9 years since I did a decent trip (and will be many more than 9 before I can do another one
above all, enjoy the trip - I can guarantee it will be a physical and emotional rollercoaster, but one of the most satisfying things you ever do!