Minuteman said:
BR816L125FDP means that it is a BR type that holds eight 1" or sixteen 1/2" breakers (or any combination there of). The L means it is main lug, 125 amp, flush mount. (I forget what the DP stands for)
QO is a type breaker made by Square D for their panels. It will not fit a BR panel. Most of us here would agree that it has a better track record than most other breakers. It cost a little more. It depends on what your customer wants and what you want to stand behind.
The 125 amp panel in the picture is legal with 100 amp wire (at least #2 aluminum) as long as the wire is protected with a 100 amp overcurrent protection device in a main panel or disconnect. If this is a main panel without a main disconnect, than there are other issues.
But, by all means replace the WHOLE panel and the double lugged breaker.
Minuteman brings up a good point. If you replace the breaker panel, you may have to bring the service up to code too. If this is the Main for that apartment, and is not located near(est) the point of entry of the service cable into the house, the service cable may have to be replaced too, (or another grounded conductor added if it's in conduit) plus a outside main disconnect added. You also may have to change the grounding system.
IMO, if you can find a replacement buss for the Challenger panel, you can replace it and not have to change anything else since the rest of it is existing. That would be the cheapest (but not the best) solution.
If it were me, I'd sell a new panel any any other work required to bring it up to code. Panel brands are like automobiles, everyone has a favorite. All of them work (some better than others) and will get you where you're going.
I usually use Seimens or GE, with an occasional Square D Homeline thrown in (price sells).
If I were doing a lot of commercial or industrial, I'd probably use QO because IMO it is more "heavy duty", and is priced accordingly.
Some may disagree with this opinion, but my 2 cents worth for free is still a bargain..
steve