The Keyspan adapters are considered one of the best and most universal ones and most of the charting software vendors recommend them specifically. I use cheap $6 generic prolific adapters with no problems that aren't supposed to work at all with Fusion or Parallels or any of the charting software - go figure.
Are you sure you need a separate serial control cable? I don't know if the llE is different from the ll, but on the ll there is only one serial cable from the modem to the computer that controls everything. There are two cables between the modem and the 710, and control from the computer is passed to the 710 through the modem serial cable. It might be different on the llE.
Mark