Yes, you can use barding. Armor in
3E only really has two forms -- for humanoid shaped creatures and other. Your companion would not be a humanoid, just as a horse, so the cost would be 2x,
iirc, check the table in the armor section to be sure. If it's a larger size than medium, the armor will also cost more.
Not sure about items specific to animal companions other than a collar in Magic Item Compendium. I recall a wizards web article on what body slots of items different kinds of animals could wear, you should try to find that.
If you're REALLY set on buffing it, one of the things to do would be get the Exalted Companion feat from Book of Exalted Deeds. Getting the feat requires being paladin-level good, though, (and as a druid, you'd have to be NG alignment to qualify), so it is likely not worth it for you. That gives the celestial template at the cost of -1 effective druid level (similar to how getting a stronger animal works).
Generally speaking, you'll want to upgrade your companion for a stronger animal each time you can, because the xtra companion benefits for a black bear, for example, do not make up for the differences you'd get with a brown bear.
For class, check out the Totem Druid variant class. It was in a dragon magazine, I know it from crystalkeep's classes database, page 35. Here's a link to the pdf file:
http://www.crystalkeep.com/d20/rules...asses-Base.pdf IMHO, giving up all the versatility of wildshape for a single type of animal creature (bears, in your case, I guess) isn't a good trade, but the class speeds up when you cna get better animals of that totem, as well as other goodies like free natural spell feat and eventual ability to speak while "totem shaped." I had a "cat lady" druid with Tiger Totem, and enjoyed it.