I´ve worked a few times with those subs and the 7500´s and they are very nice for the price, this isn´t a shitty system for someone starting if you ask me. About the box, i would definitly use a slot port, i would build something like this:
Box: 15" h x 31" w x 25" d
Port: 29.5" x 2" x 21" D
That would give you around 4.5 c/f box after displacement and 59SQ of port with a 38 hz tuning, i wouldn´t tune those subs lower than that because of the FS of the subs, but you never know, the time i used the 7500´s they could play lows very nice even with a stupid high FS. I didn´t put too much thinking on this box since you didn´t said the kind of music you like, your goals and car, but that should work on any daily application