If the cords are in good shape and the transformers do not get overly warm during operation they will be safe to use. But do not leave the transformer on unattended. The 8B has an circuit breaker to make it even safer. I have several 8B’s and they are very good transformers.
The engines will operate on both AC or DC. The AF engines mark DC are DC only.
The current draw test for proper operation is established on a 140” oval @ 12 V should pull no more than 1.75 amps. Pulling 4 boxcars on the same oval at 8 revolutions per minut around the oval should not pull more than 2.1 amps.
As far as transformer to use, for the money, two 8B’swould work fine. 8B is rated at 100 watts. If you want a single transformer to operate two trains the 12B 250 watts, the 18B 175 or 190 watts, the 30B at 300 watts or the 22090 at 350 watts would all work fine.
Vintage transformers are very durable and cost effective. As I said if the cords are good and no broken parts I’d use (and I do) old transformers.