Hello corazon,
Just a couple of points that you might like to consider before making your decision.
Your existing Lucas distributor is of excellent quality and once fitted with the Pertronix kit (assuming the kit does not give problems) will continue to function for many years to come.
The aftermarket Lucas type electronic distributor appears to be a copy of the Lucas 35DLM8, which as the ebay add says were fitted into 3.9 litre engines, typically in Land Rover Discovery and Range Rovers. Being a replica of the genuine Lucas, the vacuum module will be in a different postion on the distributor, as it is necessary for clearance of the taller timing covers found in the four wheel drives.
One final point, the location of the electronic module on the side of the distributor. As with the genuine Lucas distributors, the location of the module is the same, but unfortunately with the Lucas distributors the modules would suffer heat damage, the vehicle would stop and the only way to get going again,... the modules had to be replaced. In the next series of distributors Lucas relocated the module away from the distributor, and heat problems became much less of an issue.
If the Pertronix is highly heat resistant as from feedback it would appear to be, then that may in the long run be the better option.
Just food for thought.
Ron.