Considering first fixie

Well firstly, the advantage of using a fixed over a freewheel is that it improves your cadence so when you do ride a freewheel, you cycle much more efficiently as you are used to moving your legs all the time, and don't coast so much, in this regard it also means you never stop pedalling so are using your legs all of the time.

(Don't quote me on the cadence remark, this is what I was told by my boss when I picked up my fixed after having my Ribble stolen, however he's fairly reputable and I've had some other fixed users make similar remarks so I'm inclined to believe it)

Personally, I use a front and back brake on my fixed, braking with your legs still alludes me and I've heard it can damage your knees and it certainly does nothing for your tires, so I'm inclined to say go for two brakes. Then if you feel confident or if you really want to, remove the others as you deem necessary.

With the corners thing, it's just something you've got to learn to do, take them at less of an angle, you get used to it after a while, it's just something unavoidable that you learn to live with.

With your last question, I'm inclined to say that you would need to buy a fixed hub (Or get a track bike hub), because if you just use a normal screw on hub with a lockring on, your spacing will be off, but I may be wrong about this however, in this regard I think it's better to be safe than sorry.

Good luck, hope I can have helped a little,
Will.