Well.....
Hmmmm.
It's hard to say without a system by system breakdown. Do they power on? If they do, do they read discs? Could be a myriad of things, from bad cables, bad capacitors, bad pickups, dirty connectors, broken solder joints, chips that have been destroyed, or mix and match. This really is a crap shoot.
I have seen systems that were put in an outdoor shed for three years with absolutely no protection from the elements, work with a little TLC. I wonder if some of them were failed modding attempts. Opening them up will tell that. You'll need a Nintendo security bit for the SNES's and the Gamecubes.
A torx bit set for the Xbox's. The Wii's you will need the security bit and a tri wing screw driver. On top of that you will need a standard philips screwdriver set, 0-2 size preferable.
As far as ease goes when it comes to making some of the repairs. Ask yourself some questions. How much soldering experience do I have? Can I get it back together when I get it apart? Do I trust myself to do the job? If you answer yes to these then dig in. If not, you have a bunch of broken consoles to practice on. Can't break what is already broken, right.
The SNES, NES, Dreamcast, PSX, and Genesis are fairly straight forward in dis-assembly. The Gamecube and Xbox
can be a problem. As both have small plastic pieces that are easy to break. The Wii is a friggin nightmare. It is an amalgamation of different screw styles, sizes and plastic wankery. Real pain to get back together, correctly too.
That is if your like me and curious about this sort of thing. If not then, oh well
Sell'em for what you can get and call it a day.