You can absolutely play out every game if you want...
Regarding minor-league teams and rosters, OOTP15's announcement included,
"Our 2014 roster set includes all Opening Day major league team rosters, as well as the complete minor league system down to the rookie leagues." I believe this has been the case for several recent versions, so minor-league rosters for several recent seasons are likely available, too. It seems, however, that you are starting in 1976. That, I believe, would require considerable manual work... Others will likely have more info on this.
Yes, you can add players manually, if you'd like. The master.csv file that comes with the game might be a good place to start if you are looking for a spreadsheet.
Re facepacks, others will know more, but you might want to look around in this sub-forum -
OOTP Mods - Rosters, Photos, and Quick-Starts - OOTP Developments Forums - and also look into the All-In-One Mod (
http://www.ootpdevelopments.com/boar...e-mlb-mod.html).
As for the best stadium download, that depends upon what you are looking for (real pics vs non-pic images; if you want park factors included...). Maybe start here:
Ballparks | OOTPMODS.COM