Hi there,
I see you have already delved into the other threads about ARC/ Aviators Trading so you will have picked up some vibes already. On their website they state that they are often not contactable by telephone which would set alarm bells ringing for me too.
Bear in mind that with a reputable recruitment agency (my own experiences were with Contractair and were all positive) you are the employee and the airline is their customer, therefore the airline pays their fees... you do not. As was said on your other thread, beware of pay to fly schemes however they are dressed up, and do not get your chequebook out for these people on the day whatever the pressures they apply.
As to the "interview" I suggest mugging up on some ATR facts and figures...start with Wikipedia and atr.fr for instance and go from there....and as you mentioned Tanzania there is a good chance the airline concerned will be Precision Air, which has a good reputation among ATR operators.
If you check them out you will see they already have vacancies for ATR first officers on the website, so why not try the direct approach and cut out the middlemen...these middlemen have proliferated on the back of low-hours guys who they see as easy prey and willing to fork out thousands for a sniff of a job.
By all means go with an open mind and good luck but if you smell a fish it's probably just that....fishy.
SJ