My thought is always get all the upgrades(especialy if they count for the next tank and obviously dont if the tank is terrible) because the extra time allows you to work on secondaries. If you keep moving onto the next tank ASAP you will find your self at tier 7-8 or so with no secondary crew skills.
With all upgrades to the T29, it'll mean that only 2 new research options are needed on the T32 (Tracks and Turret)
Yes, get the engine, not just for the aforementioned bonus to the T32 but also because it means you'll be more mobile at getting out of trouble/getting into hulldown