Time Zero Professional is quite expensive by the time you add the modules that would give all of the functions of a TZTXL black box. The cost of the software and a suitable PC would be very close to the cost of a TZTXL BB. At least that was what I concluded. There may be some advantages to running on the PC since you can probably have a faster processor and more memory, as well as the possibility of displaying on multiple monitors.
If you just want access to the new charts, a much cheaper option would be to use the recreational TZ Navigator software, which is much cheaper. But you would not get something comparable to the TZTXL black box.
I looked into this and concluded that replacing the TZT3s or using one as a monitor with a TZTXL black box was a simpler path. But I haven't decided whether it is really worth it. I connected my iPad with an HDMI cable to the TZT3 and I can use TZ iBoat app to view the new style charts. Cheap and easy way to get the new style charts and especially the new bathymetric charts, which are quite good.