I'm not an expert on the game, but I do know that several of the things you ask about are directly related to hazard kits. Hazard kits are bits of equipment that have those racial abilities connected to them; by this I mean, the human hazard kits contain their shields (the higher the level of the kit, the more durability the shield has), the mutant hazard kits have their regeneration ability (the higher the level of the kit, the more they regenerate per tick), and the biomek hazard kits have their health bonuses (see points one and two).

Once you reach a certain point, you'll get access to a different sort of hazard kit that has an active ability instead of/in addition to the passive ability. I'm not sure if they do the same thing for all three races, but I know the biomek kit does a bunch of damage. In order to use the ability, you have to power up the kit by killing enemies. Once it's powered up, you just click the little "Hazard mode" button to activate it.

As far as crafting, it is pretty complex. Your best bet is to take a look at the official forums. There's a board there specifically for crafting. Someone also started an Auto Assault wiki page at AA-Wiki that you can reference, though it's pretty sparse right now.

With regard to human and biomek durability, they're both pretty sound. Both have more available missions than mutants do, which is a real plus. Biomeks also have quite a few repair pads, though I really don't find myself needing them all that often at level 54. I haven't played beyond 30 with my mutant, and I stopped playing my human around level 24, but I'd say the differences are fairly subtle aside from the number of missions. Commandos do have some advantages over the other tanks at early levels due to Defense Grid, and the Biomek starting highway is pretty brutal at first, but I wouldn't say that any of the races seemed drastically superior (at least not in PvE; I have no idea how they play in PvP).

Finally, the scout classes tend to be the fastest of each race (bounty hunter/agent/avenger). They're basically the thief/rogue group of Auto Assault- they're not as durable as the warrior classes, but they're quicker and do a lot more damage. Unfortunately, that's not really true in the early levels, but their damage does get better and better as they progress. Also, as far as I can tell, speed doesn't actually have a darned thing to do with engines. It's entirely determined by the type of chassis you use. The chassis determines how fast you are, how well you turn, how likely you are to keep your tires under you, and also plays a role in how much ramming damage you do. Your tires do play a role in handling, but the chassis is really a major factor.

I know there were a couple of things I didn't answer, though that was due to ignorance on my part. I hope I helped a little bit, anyway.