Apparently, the 2008 Kenwoods will not control a Keg/Phatbox, so you need to get a 2007 (or earlier) head unit.
For controlling a PB, it does not get cheaper or simpler than the KDC-MP235. It'll get the job done, and has a 3.5mm front aux input for idiot-proof connection of any MP3 player. Probably costs around $100 new.
To get more out of your PB, though, go for something that will give you at least two lines of text. That way, you will be able to see track title/artist on the first line and album title on the second line, for example. (The PB doesn't give you true ID3 tag capability.) The KDC-MP735U or KDC-X791 will give you that, plus rear USB and aux inputs, with the option of direct iPod control if you ever want to go that way. I think you can find either one of those for around $150.