I don't think throwing a Sora RD on the bike is going to improve the value. since Sora is several levels inferior to the missing Ultegra FD, RD, Wheel Hubs, and Downtube Shifters. What you have a a mutt.
If the bike was in good, all original (with Ultegra downtube shifters) condition, it would be worth $300-$350. Without seeing it, after reading a description of the current components, I'd put the current value somewhere between $75-$150.