Before taking the original wheel apart, make a note of high close to the tops of the nipples the existing spokes reach. You'll use this as a guide for chosing the new spoke length compared to the exiting ones. ie, if the spokes reach to the very to to the nipple, they are the max length and you'd go 1-2mm shorter. If they end below the slot, you could go a bit longer.
Now you need to compare the rim diameter at the nipple seat between the old and new rim. you can do this based on published specs. or by measuring down from the lip with the depth gauge of a caliper. knowing what you had, and the adjustments for the differences in rim diameters, and the usable range of adjustment will confirm your spoke calculation.
As to the spoke selection, I'm a big fan of butted spokes, and use the exclusively, except for special purpose wheels such as sprinting on the track. If the bike is doubles as a bit of a utility bike use 2.0/1.8/2.0 spokes.
BTW- if the wheel truly has 13g spokes, you might find the hub drilled a bit large for 14g spokes (also might not). if you consider the spoke fit sloppy in the hub, you can slide #2 stainless washers down the spoke priot to threading them to tighten the fit a bit. Or since it's a bit of a work bike use single butted 13g/14g spokes to build workhorse wheels.
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