Work depends on your finances. A lot of rusty parts will clean up (do a search on rust and oxalic acid). No one keeps a bicycle maintenance history, don't worry about that.
Inspect inside of frame for rust, if it is crusty, you are going to need to treat the entire frame (after totally disassembling the bike).
Replace all consumables: tires, tubes, cables, bearings, grease.
The bike will probably require a complete overhaul. If you take it to a shop, that will cost $200, maybe more. If you have the time, you can certainly do it all yourself.
I pulled my 1975 U08 which had been stored for over 30 years in a similar fashion. It cleaned up great. Somehow, the frame (internally) did not have any rust.
Are the rims steel? If so, find some alloy replacements. I am assuming they are 27 inch rims, which you can find used cheap. If they are alloy, have the shop inspect them first.
The cheapest option is to start looking for a "donor" bike, a cheap bike with similar to better components. The donor could provide derailleurs, wheels, and other miscellaneous parts.