I'd aim for level 30-50 (For a solo challenge maybe 35-50, though, I'm not sure). I myself am currently trying the "Perfecter Game", so I'm going up for the 70-80's. You can get to Level 99 if you want, but keep in mind, it takes a HELL lot of time. Level 99 could take up to a month, depending on how much you play.

Once you get the hang of it, you actually don't need to save between Dustia runs. When I started, I saved often, but now I think I regret doing that.