Here is another thread on a similar topic:
http://www.bikepacking.net/forum/index.php?topic=10432.0I'm old school and have been running 26" wheels, V brakes on a rigid or hardtail with racks and panniers for a long time. Not the fashionable setup, but it does the job. Most of my riding is paved and dirt roads with some technical, east coast single track thrown in.
Here are my opinions:
I would go with a rigid fork for reliability reasons. I would go with mountain bike tires/wheels/wheel clearance for the same reason. Also, you want your racks/lowriders to to have rock solid attachment to the frame/forks. Same goes with pannier to rack/lowrider attachment. Front low riders are nice because the load is so low, but bags can hangup on stuff on single track... a front rack moves bags up a bit. A bike that has sliding or pivoting read drop outs for single speed conversion has some real benefits if your rear derailure blows up far from a bike shop. (Emergency single speed conversion without a chain tensioner is hit or miss, mostly miss). If your bike has a replacable derailure hanger, be sure to buy a spare and carry it with you. 2 or 3 sets of bottle cages mounts is fine, you can always put extra bottles/bladders in your bags.
Bike fit is the most critical aspect of a bike purchase. If you have bike that already fits, reverse engineer the critical dimensions and use those dimensions when shopping for a new bike. Frames are proportioned for drop bars or flat bars, switching between bar types is not a good idea, so make sure you decide on what type of bars you want first.