I hold fairly extensive Dungeon Master experience, and in particular, I can safely say that while a Play-by-Post scenario is possible with Third Edition, the fluidity of Fifth Edition and its flexibility is why it holds a much safer, more readily used placed in that role. The sheer amount of math that can get involved, just by rolling the dice alone, can make it impractical as does its favoring of grid based combat rather than the aptly named "Theater of the Mind".
I will agree with you that the Third Edition has the absolute most customization possible, which is both a massive boon and a horrible curse depending upon what it is you intend, but I also hold the hope that Wizards of the Coast will be wise enough to invest heavily in Fifth Edition and continue to attempt to expand its influence and role in the market. As of now, Fifth Edition suffers from being too new and its supplements being slowly developed.
Sadly, this reply has offered little to resolve your issue and were it not such a bad time in my reality, owing to my extensive working hours, I would indeed be running a variant low magic savage fantasy Third Edition setting from my own works. In perhaps April this can be a viable thing, provided you still hold interest then, but I must also say at the same time it might not even be what it is you are looking for. As example, I use a modified E6 variant and I have tweaked some of the rules - currently having just finished the "beta" run of the setting in an actual table top - of which I am still working on.
