Definitely made the right choice going with the REmake first. Zero is a fairly divisive game. So here's some other answers to your questions:
- In Jill's campaign you only ever play as Jill. Also, Jill is a good choice for a first time playthrough. Keep in mind that if you enjoy your Jill playthrough there is unique stuff in Chris's campaign.
- Playthrough time is something I've always been bad at gauging so someone may give you a better estimate.
- The story is good because of how campy it is. Check out the original version of RE1 sometime. The bad voice acting is legendary as is the uncut intro fmv:
http://youtu.be/PwYu-LOXAWQ
- So a good habit to get into may be saving once you've picked up a new key. Or saving before you go to unlock something with a new key for the first time. What I mean is, if you picked up new key and you know of multiple doors it will unlock then maybe do a save before unlocking the first one of those doors. After awhile you'll start to get a feel for when a good time to save is.
- Only kill what you think you can't get past without taking a hit. Otherwise, just try to avoid enemies. Keep in mind that you can kind of manipulate zombies into moving where you want them fairly easily which should help with navigating the mansion without taking too many bites.
The REmake made a big impression on me back when I played it on the GameCube. I'd be curious to hear how your playthrough goes and if the game holds up for someone going in with no prior experience.