Hey there!
I mainly posted because I've been looking at supplying Kelestia with more D20 Harn material. My understanding of D20 has improved over the last few years, and I think the campaign setting is one that people should look at.
There isn't a definitive D20 set of rules for Harn. Shane Morales did a good D20 Harn document (a google search will turn it up), but it was 'fit D&D into Harn' and (IMO) didn't capture the flavor of the campaign setting. Other people have used Iron Heroes or d20 Conan as rule settings to try to get closer to the Harnic feel.
Harn itself isn't 'low-magic', it's 'rare-magic', much like Middle Earth. Anyone who has played Harnmaster will tell you that Gulmorvin are scary undead creatures, accomplished mages can travel to other planes, the sindarin are powerful beings with ancient magic, there are dragons and undead and evil wizards, and the god Ilvir himself lives in a foul pit on the island of Harn, and creatures that he spawns are the stuff of nightmares. But it is a very gritty, survival oriented setting, and the people of harn are ignorant of much magic out there. I'd describe it more as a 'dark ages' setting.
Since Harn was originally published as a 'rules neutral' setting, there's no reason D&D won't work with Harn, as long as the feel of the campaign setting is addressed. D&D is primarily aimed at high fantasy, but it can support other grittier settings - Iron Heroes, Conan, and even Ptolus show that.
I'm not familiar with Legends of Dark Ages or Legends of Excalibur. What are they like?