Echo everything BobCW said. I would add a few points however:
*If you are focusing on MP balance, go with
Erebus in the Balance. I am a little biased, as I have taken over development of it, but it is the only mod being developed with MP PBEM in mind (v11.1 should be released in the next few days). If you plan on fielding an significant amount of AIs, then go for ExtraModMod, which rolls most of the balance changes of EitB with some AI upgrades and its own tweaks. Regardless, I would strongly recommend using
a modmod, as FFH2 as is suffers from considerable problems with AI and balance.
*I would strongly underline the points on quick speed. Anything else is masochistic in a ancient-era start, especially if you can't mantain a 1tpd schedule. If you must use normal, consider starting an era in.
*By the sounds of things, you look like you are considering a large number of players. I would recommend sticking with 4-6 players if you keep it PBEM, as anything more and turns begin to be missed regularly. If you are prepared to use pitboss, then you will need EitB (or a modmod that uses the fix the reddit community found), a host, but otherwise you can play with a lot of players with only a couple of balance issues.
*I would recommend against populating the map with AIs. You can do it - but it can increase the distance between players and reduce player-player interaction considerably, and is also a very serious balance issue.
*Consider Full_of_Resources mapscript, to lushen up a map, if you intend on using a purely random map. (I can look over a map for you, for some small balancing)
Depending on how the settings pan out, I may be interested in playing this. (DON'T consider this a signup yet, however.)