Yes, it is SLX, which would put the bike no earlier than 1984. The number under the BB is not a serial number. There were no serial numbers on them at that time.
You may be able to get $1200 by selling everything separately, hard to say. I wouldn't say the condition is very, very good, more like fair to good, if there are no dings or dents. It appears to have lots of paint chips on it. De Rosa measured the size by the seat tube, c-c, so 58 would be correct, if you measured it right.
The missing correct components, and the wheels hurt the value. I don't think you could get $600-$700 for the f/f in this economy, but who knows? Only way to find out is put it on eBay, and be prepared to pay the fees. They are nice riding bikes, if it fits you, you should keep it.