1) Stay with the Dallas 80C320 or convert to a a Cygnal part. We have that perogative because the project includes a new PC board.
Im sure the Cygnal is a fine chip; however I would say that the most important you can do is add ISP. I do not know if Cygnal has this.

2) Use the Nohau emulator or find another solution. This unit is 1994 vintage and may not even be supported anymore. I have used Nohaus of that aproximate vintage and they do the Job, the only problem you may have will be a possible incompatibility in the current debug format of the .hex/.abs file and that vintage of Nohau.

3) And finally, is the Keil tool set the optimum one to purchase.
I would, when I have not used Keil it has always been because of wanting the same toolset for the '51 and another processor not supported by Keil.

Erik