Very welcome to this site! I hope that you will find many good contacts, stimulating discussions, and many useful things here.
I am not sure which alto guitar would suit you best. Myself I have two 11 string alto guitars tuned in the usual way, only with the difference that the third string of one of them is tuned half a note higher (guitar tuning). I use my alto guitars for many types of music, but mostly for renaissance and baroque music. Other members of this site will certainly give you more information on this matter that you can use while making your choice.
Did you know that you can find a lot of sheet music here? For instance, the entire book Altgitarren is here, and you can get it following this link: http://altoguitar.com/index.php?option=com_fireboard&Itemid=31&func=view&id=1626&catid=16
Then follow ninjasan's link in the third post of the thread.
Best regards, and good luck in choosing your alto guitar!