Whatever you build try to keep these general guidelines in mind (these have worked well for myself and many others):
Whatever you build/however you build it, build it as predator proof as you possibly can (and in my experience ground level buildings/coops work best, rather than elevate coops that need ramps to the entrance).
4-5 sq ft of floor space per adult bird keeps everybody happier, and squabbling to a minimum.
Mount plenty of roosts as high as you can that will still allow the birds a safe soft landing on deep bedding, but not so high that they are going to bonk their heads on the ceiling).
Making sure it's well ventilated but sheltered from drafts/harsh weather helps keeps your birds healthy.
And an attached covered run on the coop is a priceless addition/tool. Especially for the days you can't let the birds free range or need to have someone take care of your flock for you.
Hope that was helpful! Good luck with your Guineas!