The main board is obviously the most complex of the P3 boards, but assembly is not too difficult if you follow the usual process - start with the smallest components, i.e. resistors and diodes, then work up. I use a PCB soldering jig with a foam-lined lid to hold all components tightly in place while soldering. If you build a lot of kits, I'd highly recommend investing in one.
To save time and money, I don't socket every IC, but I strongly recommend you do. It is essential the CPU should be socketed.
For reference, here is a fully populated mainboard (click for a larger image):
There is only one optional component on the v1.6 mainboard - a backlit LCD display module requires R19 to be fitted. If you use the standard PLED module, this resistor is not required.
The preset resistor VR1 adjusts the brightness of the PLED display, or contrast of an LCD display. For a PLED module, just before the 9 o'clock position is a good starting point.