1) The documentation isn't that bad if you just need to look up syntax or the general gist of what a command does, however in terms of learning the language the help files don't help so much. To start off with the language it would be best to either get the DBP books(I've not read them) or look at some user made tutorials (the Binary Moon hovercraft one comes to mind). I personally started using DBP at the age of 12(my first language) and it took quite a while for me to understand most of it as I'm not so fond of following tutorials and prefer to learn by myself, I started off by modifying examples and looking at other source code. I find this approach fun, but it's slower in the long term I guess, and you can learn bad coding habits(not that the DBP examples will make them any better

).
2) I've only recently bought a few plugins as I've dived into making more complex / large projects, however you can make do with just DBP(with patch 6.2) and a few free command libraries, you only need things like Cloggy's D3D DLL and Sparky's DLL to start off with(both are free) and other things like Benjamin's Tempest / Multisync(also free). The only plugins you may need are EZ Rotate(unless you're a math genius

). Things like physics can be done using Walaber's Newton wrapper(free) AI is probably the only snag as IanM's A* library while very useful is merely a library and not a complete package like Dark AI and I don't recall any user made plugins / code that do something similar.
3) Never used Torque, however DBP doesn't come with much in the way of tools on its own. There are many user made tools available that are usually free, things like a modeling app can be free if you use Blender for example or GIMP for textures etc, so if you don't want to spend much you don't have to.