How far off a chain line can I go? I'm struggling to find a replace crank set to replace the SR Suntour set that came on my bike. The chain rings were riveted together so on bending one I had to replace the set. There's very few 7 speed crank sets for a 118mm spindle and separate chain rings. I would like to fit a Shimano M410 on a 113mm bb. This will alter the chain line by 2.5mm, will this be an issue?
2.5mm would not be a serious issue.  You may find the "sweet spot" in your gearing changes a little to favor the larger rear sprockets, but so long as you don't run a lot in your top-most gear this should not be a problem.