From someone who has built a ton of stuff out of that pre-bent stuff, just buy U-bends they are far more universal. Proper shape and where it goes is just trial and error, after a few try's you will start to get the hang of it. As far as the stainless goes... Well I hope your a decent welder, 1/16th stick will probably be your best bet, as most wire for stainless is fluxcore and doesnt like thin stuff. If you have a Tig that is the easiest and best, but few people do. Good luck.
The other thing you can do is cut it and tape it all where you want it to go and then take it to your local welding shop and have them weld it for you.
As far as mateing it to your engine it depends what you are building, if it is an intake or exhaust manifold then you will need a flange that matchs you intake or exhaust, if it is for a downpipe then you will need the flange that bolts to the turbo you are using.