Unfortunately the parts mix and missing original fork hurt value... I would say part it out.
I can't really put a solid value on the frame though, unfortunately. Depends highly on the market, also urban vs metro. I've seen prices for SL/SLX framed bikes vary from as little as $300 to as much as $1000ish for bikes in amazing shape.
You could try listing it before you go through the trouble of parting it out. You just never know who will see it and this might be exactly what they're looking for. Maybe ask $750 and see what kind of offers or interest you get? I will say that 59cm is pushing "big". Any time I fix up a bike that size around here (Dallas) it always sits a bit longer.