SD5 arms and lever with a set of decent plaz pads and they will be better than most maggies. My front brake with that set up is the most powerful brake I have tried.
+1 I use a Taya singlespeed chain that costs £5.99 from Halfords. It is a beast I usually snap chains easily but this ching has lasted 3-4 months though I suspect it will need changing soon. (I can usually tell at the start of a ride whether my chain will break or not).
If you don't know what you are doing take it to your lbs when they are not too busy and ask them to show you how. Better that than f*cking up your hub!
YES!!! Shimano will never give you problems but they are a bit under powerfull for trials. Try a hope or alternatively (and I do reccomend this) try using an avid mechanical as they are so much better than their hydros.
You have 135mm spacing yes? What I would say is the echo is good but it is often worth going better. Do you want to run gears, if not I would say screw hubs are the way forward. Have a look at the viz screw on hub here.
Handlebars and stem for free. Pedals for free. New BB7 for £10. Grips for £5 (should be £15). Hope moto V2 for £40. Planet X forks for £55 when they were marked up at £100. Free innertubes.
20" is very good to learn on. I personally prefer 26" but either is good. Mod is easier, especially for spins, but 26 is in theory easier to go higher on.
It's square taper I take it. Pretty much as above bit I generally put four stips length ways (one on each side) rather than one around. If it is isis it is possible to do but it takes ages and often will only last for a shortish ride before it needs redoing.
Around 20psi is the norm. Anywhere from 10-50psi will be fine depending on your setup, style and what sort of stuff you ride. I personally recomend between 20-30psi! The best way to find the best pressure for you is to try loads of different pressures until you find something you like.