This is just another case, where the T.O. should at first think about, and define/specify the goals of what we wants to achieve.
Whenever I read ".. want to set up my own small CAL LAB.." and then directly ask for 8.5 DMM , that's the wrong approach, upfront.
Make yourself clear that, if you REALLY want to do electrical calibration, you at first need all kind of sources, i.e. for all different modes and ranges of a typical DMM. If you own either the 3458A, or the 8508A (or similar), these alone will not let you calibrate anything, because all the (stable) stimuli are missing...but both instruments are practically on par for most of the calibration tasks, as soon as you also have the appropriate calibrator, as TiN already stated.
If you just want to maintain and measure only the basic electrical units, (not to really calibrate anything), better get the 3458A, a 10V and a 10k standard, and you are done. ...because it's got the unique Artifact and ACAL features, which lets you precisely adjust all modes and ranges from only two sources.
And again, make yourself sure, which units / ranges you really need / want to measure, and to what accuracy.
And plan for cost for regular calibration of these instruments also.
Frank
PS: branadic, please do not exaggerate the vintage of the 3458A, it's from about 1988, so JUST 30 years old.