600RX is a great strobe, but also requires an additional RX receiver to work with Skyports. I went with the BXRi series because of the integrated Skyport and because I didn't need the slightly faster duration or slightly better build quality of the RX strobes. The newly released BRX strobes look even better than the BXRi, and are probably what I'd choose today. For in the studio, lower power is often better especially with 35mm digital. Consider the 300RX or the BRX(i) 250.
Honestly I think given your kit I'd put money towards lights next. You've got a good range of glass including a nice fast aperture prime. The addition of strobes will make a far bigger difference in your photos than the subtle changes of image quality of going to L prime lenses. One good light, in a portable package can be very beneficial and make a drastic difference in the look of a photo.