No.
does Casper have a minimum required deposit to become a validator? For example, 50 ETH, 1500 ETH, etc.
1500 ETH currently.
I’ve seen research suggesting, if I understand correctly, that interest on deposits will fluctuate up or down according to proximity to a desired Total Deposit target. Is this confirmed/accurate or are there other strategies?
No, though we have explored the possibility of mechanisms where the minimum deposit goes up as the number of active validators goes up.
is there a set withdraw limit on the deposit, ie held for four months minimum?
The entire deposit is held for 4 months before it can be withdrawn.
do I understand correctly that interest, and rewards/penalties generally, are dispensed per epoch?
Yes.
do answers to the above apply to both Casper FFG and CbC?
Not sure; ask @vlad about CBC.