Re: Who's eligible to teach Aikido? And what level a sensei should be?
If you are worried about a fraud there are a number of things you can do. First ask for his Aikido lineage (who his instructors were and their instructors and where he trained). Aikido is such a relatively new art that we can still trace instructors back to the founder Morihei Ueshiba, also called O'Sensei. Ask which affiliation he is certified through. That should give you enough info to verify his credentials.
I know that "Real Aikido" or "Realnog Aikidoa" can be found in that part of the world. Though it includes similar techniques to Aikido I can not find any connection to O'Sensei (if anyone has information contradicting this could you please provide a reference). It appears to me what the "Real Aikido" practitioners do is effective technique, but I can't say for sure without having practiced with them. Make sure you are getting something that can be traced back to O'Sensei.
Personally if they are at Black Belt level be it shodan or godan I wouldn't see a problem training under them. But the real test is to watch their students. Can their students do the techniques well? Do they continuously make mistakes that never get corrected?
Ask if they give a free tryout class. Most good schools are willing to give a free class or two so that you may see if you are interested in the art. During this class pay attention to how he teaches his students. Is it a teaching method that you are comfortable with? When you are uke do they take your balance and then apply a technique? The Aikido techniques will be very different from what you are used to in Sambo but when you feel them you'll know if its effective or not (I came into Aikido from Judo).
Based on my own training I can recommend Iwama style if you can find it or anyone that can trace their teaching through Morihiro Saito Sensei or Shoji Nishio Sensei.