I just finished building the K3 with nearly all the options including the Sub Receiver. I enjoyed it for the most part. However, there were some places that were really difficult to get my hand into a cramped area.
So if you are like me and find XL gloves to be tight, you might want to think about getting it already assembled. Oh, and make sure the sub-receiver is the last thing you put in.
The P3 was no problem. If you get the SVGA card, plug it in during the main build vs following the instructions that were written for inserting it into a P3 that has already been completed.
I am 6'6" and about 275 and have a hard time fitting into XXL gloves... Glad it wan't just me.