120 mm hubs can often be respaced for wider dropouts, you need to check if it comes with a long enough axle, or if the axle can be changed. Also, you are right that ones for indoor use may not have seals suitable for all weather riding, so check that too.
If you want double-sided, don't worry about the difference between fixed/fixed and fixed/free. Not using a lock ring probably saved me from a lot of grief recently when I lost a side-plate from the chain, causing the chain to unship the wrong side of the cog.
You ask for the "best option", that is almost certainly goldtec. But note it is the same hub at the different widths, they are just fitted with different spacers on the axle, so it won't make any difference in the wheel strength. They do sell the spacing kits separately (fairly pricey tho), so you can easily switch it to your custom frame when you get it.