The Nexus phones are generally fairly cheap and are updated regularly. I'd almost suggest a Nexus 7 tablet, though it isn't officially (key word) compatible. I use my Nexus 7 tablet for viewing the clan wars while playing. You can get one for $200-250 new or cheaper if you can get one refurbished or used. Phone wise you could try to get a Nexus 4 (used). Not sure what other phones out there are really good and are being updated regularly. You can trust that the Nexus devices will be updated for a while unlike some devices put out by other companies.
Thank you for taking the time to post! Please check our list of compatible devices: Activision Support in that case you will be able to verify which one will fit your budget. In regards changing your Clan Slogan, first enter to Clan Settings and then to Clan Motto. That should allow you to change you Slogan.