Quick comments:
1) DMG contains Epic stuff, so Epic is technically "Core".
1b) However, we're in House Rules, so "Core" shouldn't be an issue.
2) Setting DCs for Nifft's Scaling Skill System, Example 1: A specialized Rogue.
Assume Int 14, non-human (so 10 sp/level, 40 sp at 1st level).
Level 1: 10 skills at rank-4.
Level 5: 5 skills at rank-4, 5 skills at rank-8 (or 20 skills at rank-4, etc.).
Level 9: 5 skills at rank-4, 2 skills at rank-8, 3 skills at rank-12 (+1/level left over sp).
Level 13: 5 skills at rank-4, 2 skills at rank-8, 1 skill at rank-12, 2 skills at rank-16 (+2/level left over sp).
Level 17: 5 skills at rank-4, 2 skills at rank-8, 1 skill at rank-12, 2 skills at rank-20 (no left over sp).
Level 20: 5 skills at rank-4, 2 skills at rank-8, 1 skill at rank-12, 1 skill at rank-20, 1 skill at rank-23 (+4/level left over sp).
IMHO, it's not unreasonable to have one or two skills at level 13 with which you are simply better than everyone else. It's clearly a big deal to specialize this much. (In Core D&D, you'd get to "specialize" in 10 skills -- not quite the same thing.)
So, setting DCs: since skill ranks will on average be lower, you can get away with significantly lower DCs. For example, what was once a DC 94 check should now be a DC 34 check (!!!) (ranks for 84 skill points + 1d20). You can leverage the chart that I made for skill points -> ranks as a cheat sheet for old DC -> new DC -- just don't forget that DCs are always about 10 higher than the skill they're checking.
Since you specified that this is a low-magic setting, we don't have to worry too much about skill-enhancing magic items screwing up our calculations. My advice is to grant a few +2 to +4 skill boosting items, and charge a lot for them. That +2 and +4 are worth up to +14 and +28 skill points (at 17th level and up), so the bonuses clearly scale well in "Epic" rules.
So, my advice on skill items is divide the bonus by 5/2 and raise the price.
Let us know how your game goes! -- N