Having a T/S or T/C is a must, I would prefer to have a second altimeter before a second horizon; once, on IMC departure, I had my primary altimeter freeze at 500ft while climbing at 1000fpm IMC; second altimeter saved the day.
You need to practice partial panel from time to time, even on a simulator is better than nothing. It is best to have some device to cover the offending horizon because flying in real IMC with a failing and tilting horizon is disturbing.