You probably won't need an AI at all at 250mg. The question is why only use 250mg, it will shut you down as much as 500mg will do. 125mg is pretty much TRT levels and is pointless for performance enhancement... If you are going to do a cycle then you might as well do 500mg a week.
HCG should stay the same thats basically a minimum dose. 250 iu HCG pretty much maintains intratesticular testosterone levels
https://www.ncbi.nlm.nih.gov/pubmed/15713727
Good choice on the exemestane. Harder to go wrong with it as it has a limit to how low it can take your estrogen and you don't get an estrogen rebound once you come off it.