First Build, some doubt (Prusa i3) (4 replies)

Hey, i'm almost done with the list of components for my first RepRap but i've some doubt, mainly with the extruder and his support.

I'm thinking of builing a Prusa i3 Boxed using this printed parts ([reprap.org]), but i was thinking of using this ([www.thingiverse.com]) x-axis carriage insted of the standard/old one (the one that can be found on the page with all the other printed parts).
For the extruder i was thinking of this one ([www.thingiverse.com]) with a j Head MKV.
In addition i'll probably add this ([www.thingiverse.com]) fan support for cooling the barrel,

Now here comes some doubt =)

1-It's my configuration correct? I've researched it and it seems fine to me, i presume that i can exchance the x-axis carriage without problems or am i wrong? For my understanding the original versione is the one for the Boxed and the one i would use is the one for the single plate design.
2-There is a better carriage/extruder configuration that comes to mind?
3-Last thing, the fan/fans, it's not all perfectly clear to me, i've read that there are 2 parts that benefit from dissipation, first the barrel, for preventing the heat to rise on the extruder printed parts and melt it, and second the print, for various reason, am i right?
Any suggestion on how i should configure the fan?

Thanks for your time, hope you guys can help a newly discovered Reprap enthusiast!