How easy to replace laptop motherboard?

Swapping motherboards on a laptop isn't all that difficult, even less so since you've already managed to remove the faulty one. I personally would have sent it in for repair but I assume that your warranty is expired.

In any case, I can't tell you what you will need to carry over because it will vary from system to system. On some systems the CPU is soldered directly to the motherboard, on others it is in a socket. The same goes for the GPU, wifi card and HeatSink/Fan assembly. The RAM you will definitely have to remove and carry over (I hope that you removed it first!). Ultimately you will have to wait until your replacement motherboard arrives to tell exactly what needs to be done.

Once you put it back together it should work just fine, but it is possible that the firmware on your new motherboard will not be the same as it was before, you should check this ASAP.