If its flexibility you want...
then perhaps a revived Energia program would make more sense than a modernized Saturn V. The Energia was a marvel of growth potential and payload flexibility.
Odds are the Energia could be revived a lot easier than the Saturn V, too. The last Saturn V was launched almost 30 years ago. A high percentage of the knowledge base that built, serviced, and launched it has either retired or died. The facilities, tooling, launch pads, and support infrastructure for the Saturn V are all long gone. Nostalgia shouldn't be the basis for a national space policy, as wonderful as it would be to see Saturn Vs flying once again.