Since 5.0.5 or 5.0.6 the original Microsoft diskette installation procedure does not work. The easiest solution is to tar /usr/lib/foxpro and /usr/bin/fox* from your existing system and then restore the tar file on the new system (either 5.0.7 or 6.0.0).
If you are planning a fresh install anyway, there is little difference in the amount of effort required for either upgrade.
The certified hardware list might become the deciding factor.