1) You can easily check all available units for all factions in the Recruitment Viewer program, which I believe is included in the EB install nowadays. The installer is in the Extras folder.

2) IIRC Armenia was not a unified kingdom like the other factions. The lvl 3/4 represents the challenge of uniting a divided tribal kingdom and the investments needed to do that. In a very good and fun way I might add.

3) Some prefer going all HA using the Scythians from the Kartli province. I usually try to build a lvl 3 native barracks to get a production of Native Phalanxes. Then I use bridge blocking/gate blocking/any other cheesy tactic that keeps me alive. If you have the manpower you should try raiding into the AS heartland. Seleucia and Babylon have a lot of expensive buildings, a lot of people to massacre and usually are very underdefended. If you are lucky you might even get away with keeping them for yourself. I usually leave again though. Let the Greeks pay for rebuilding while you spend your cash improving Armenia.

Good luck to you. If you fail, don't give up. Hayasdan is a difficult faction but they are imo the most fun one as well.