I think the highest reduced level is 18 for lvl 60 items. There are lower level items with reduced level as well, but I'd skip those. The ideal level to get a high damage lvl 60 weapon with reduced level is at level 46-50. Makes the leveling up process tons faster.
1) 42 is the lowest level to get them (so level -18 reduced).
2) Level 45-50 is when it's cheap to get a good one. I'd actually suggest level 50, since that's when you start to run into harder stuff and the price drop dramatically. Stuff at level 42 that are good are worth just as much as good level 63 gears.