There are many apps that have file sharing like Files, Drop Box. They use either WebDav, Samba or whatever. Since I have the files on a remote server, the Ipad will play whatever without the need for synching. You can get one of those USB wi-fi NAS and store all your media on it and the Ipad will have access to them. I have about 4 terabyte of content accessible from my Ipad. It works. You can put a 1 terabyte NAS in your trunk and you are good to go.
AT&T does have an unlimited bandwidth for $30 a month. $15 for 250MB.