Not knowing what the original recipe is, I'd say start with 1lb of whatever sugar you choose (that should be around 10%). If you decide to go with maple syrup, I'd get the real stuff, not the maple-flavored corn syrup in the pancake bottles. Otherwise both brown sugar and molasses will work as well, but if I had to guess I'd think that molasses would lend more flavor than brown sugar, unless you can find some good dark brown sugar. It kinda depends on which way you want to go with the recipe, but I don't think any of the options are bad.
If you're making yeast starters and have a decent method of ferm temp control, then I'd suggest just adding the sugar to the boil like extract. Similar to extract you'll want to make sure they get stirred in well so they don't scorch on the bottom. If the recipe was higher in gravity, or you tend to ferment towards the warmer end, you could consider adding the sugar after a few days of fermentation by boiling it with some water to sterlize and then after cooling adding it to the fermenter. I think from the sound of your recipe you'd be fine with adding it when you add your extract though.