Hi, would you buy a second hand DMM ?
Depends on what you expect and need. Want warranty and support when needed - buy new.
When is a DMM to old to buy second hand ?
There is no such thing as too old. You can use 40 year old nixie tube DMM and it can be still better than brand new cheapie
. But older gear need more care, and servicing more likely than not.
Can it have a lot of problems ?, how do i know its good ?
Of course it can have lots of problems, from tiny ones like bad cable contact to the huge clusterfails like unstable custom hybrids, unobtainable ICs, burnt holes in PCB,or hidden intermittent issues.
To know it's good you need to know what DMM can do and cannot, and have other (often expensive) equipment to be able test that multimeter. Performance test procedures usually listed in user's manual, however even handhelds rely today on multifunction calibrators like Fluke 5520 or 5700A, because many ranges and many functions available. Since you asking questions like above, you unlikely to be prepared to know how much those cost
.
What does new DMMs has that old ones dont have ?
Fancy LCDs, touchscreens, Bluetooth and other rubbish, usually not related to DMM function directly.
Old ones from top tier brands usually better built, have nice beefy chassis (which can be PITA to send/haul over the lab), often many discrete-based circuits instead of two chips with custom firmware. There are cons and pros either team, pick what you need more. Some of old DMMs (designed in 80's-90s still kicking ass of any new DMM for specific purpose uses).
I better buy a new one to stay out of trouble ?
Even if you buy brand new meter, trouble still can get you (bugs in linux firmware, poorly designed cost-down circults, EMI/RFI issues, etc).
What is your very accurate voltage source, that you looking into 5.5-7.5 digit meter? (You do understand there are 10-20 times price difference between 5.5 and 7.5 meter, right?)