Would not be concerned about “screwing them up” by plugging them in, as they do not work now and need repair anyway, But the safest way is to use what is called a “Dim Bulb Tester”. Basically its a Incandescent light bulb, usually 60 Watt, put in series with the item under test and if there is a short within the radio the bulb will turn on full brightness, if there is no short and basically in decent shape the bulb may turn on just Dim or may turn on bright for an instant and then slowly go to Dim. Do an internet search for Dim Bulb Tester for complete instructions and how to build one, its a very handy item for the test bench and can save you a lot of fuses and other problems.
As far as being Rescued, that all depends on the owner and how much they wish to spend as far as parts are concerned and time, most anything can be rescued. Even if some parts need to be machined it can be done but there is a cost factor as well. but the choice is by the owner
As far as the best source for learning….hard question to answer but there are a lot of You Tube Videos which are excellent but also some which are total nonsense, also electronics online courses, but keep in mind that anything with TUBES …. Transmitters…..old TV’s and similar items have LETHAL VOLTAGES inside and extreme care must be taken as DEATH can be a touch away. I had an Audio Amplifier brought in for repair (tube unit) and it had not been turned on for at least 6 months I was told and when I put it on the bench the power supply capacitors still held a full charge of 350 Volts DC which really was not expected but I checked it with a voltmeter before touching it, these particular Capacitors did not have what is called a Bleeder Resistor on them, I did install them for safety purposes.
Now the transistor items and battery powered items are not as dangerous but all electricity has its own hazards and best to avoid any shock but we do on occasion find ourselves getting a shock even when experienced with electrical work and when that happens we Learn from it ….Hopefully !
There are also many publications covering electronic repair/work, home study basic electronic repair courses are offered by some company’s, ask questions at a local electronic repair shop etc….
Hope this helps