can series 4&5 rear diffs n stuff fit series2.

my mate crashed my rx so i thought i might as well upgrade it. i will also have 5studs.

no you cant, irs and rear live axles arnt interchangeable, you can find a way to fit the hubs, but it will require machining and customising.
some miata lsd fits in s3 rear housing, but i dont remember if its only in 13b first gen they fit (gsl-se)