Three of my 34401A (year 1992, 1995, 1998) did not drift much. I bought all of them used, burn in for 4000 hours then sent it in for calibration. They were bought separately many ages ago. They stay in spec till today. I use three LTZ1000 powered as my references. They are also burned in for 4000 hours then voltage verified at a lab.
If you want absolute stability and very little long term drift, get the Keithley 2000. I have one used and it stayed true to the last digit for the pass 5 years.
The slow boot up of the 8845A would not be a problem if you run a lab most equipment would be on for long hours. 8845A is a newer device compared to the 34401A. I prefer Agilent products the vfd module is plenty for $100 if it dims after a few years.
tapatalk