Should I really build my own stage1 or just use one from a Gentoo mirror?
It is best practice to always build your own stages if you are not using the same snapshot that was used to build the last release. The state of the tree is very much dependent on itself. If you have a stage1 from 3 months ago, it is very likely that you will run into problems with blockers and other changes in the tree that will cause you build or compatibility problems.