no the psu included in the build is more than enough even for the most powerfull single gpu's available, dont be fooled into thinking you need anything more than 500-600w, usually 650w is enough for crossfire nowdays, so dont worry!
and as for building, it often gets referred to on here as "easy as building lego" most things only fit one way, there is also a build guide on the forums, and if you ever get stuck just ask everyone on here. we are all more than happy to help.