2000er KA with 67tkm 1.Hd. rust on tank and underbody!! What can cost complete renovation???

Hello Ka fans, my wife has a really nice, black, deep, wide KA EZ 2000 1.Hd. and except for the rust around the tank cover is the visible condition TOP. Yesterday she was in a free workshop because the ABS lamp did not go out. The said handbrake is stuck and the demanded brake discs and coverings are gone. The exchange would be ok, but now comes…whether she wants to invest that because of the strong ROST in the underbody and chassis area. I have already read something in the forum, but have not seen any prices of a complete underbody renovation. My wife would like to drive it even longer, has ersma still 1 year TÜV !!! FOR RESTRUCTION AND TIPS TO DESTROSTUNG BZW. PARTAUS-TAUSCH, WNN MÖGLICH WITH CA. PRICE, WAREN WE EUCH DANKBAR! Greeting, MC