Subpanel questions

wease

My current panels are 60's vintage. Two of them 200 amps each. I believe it's armor clad wiring and I can't buy breakers that fit these anymore. I can find them on EBay though. So two questions:

1. If I bought a big breaker on Ebay can I use it to feed the subpanel?

2. How big of a breaker to feed a subpanel that will feed about 18 circuits covering 2500 sf of living space? 120 Amp good enough?

Or could I just replace one of my panels and feed the power directly to it?

KUIPORNG

I think you may as well replace the whole thing with modern panel which use modern breakers... for that... you need to hire someone to do... I believe most code does not allow DIYers to tackle it themselves as the contractor need also to coordinate with the utility company to turn off the main switch..etc...