I found myself today with this, for free, an Adler Electric 21 (i am not sure if this is a 21C or a 21D ). It is completely filthy, but it wants to work, I tested it and some keys do move as intended, others move once but the carriage does not advance. I have so many questions since I was not able to find any technical manual or documentation for this model. I might open a new topic documenting my repair adventures for this behemoth.
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It has come to my interest the acquisition of an Underwood typewriter, particularly a model 3 o 5, models which are not necessarily common at my location. I am hesitant to take the well-known risks for buying online in not specialised places (overpricing and bad packaging), therefore the reason of this post/buying order. 

I can accept machines in general suboptimal conditions as a restoration project, for which I have guidelines I will detail next.

- Dirt and gunk: Accepted, even if this has halted to some extend the functionality of the machine. 

- Rust: Acceptable as long it does not compromise the structural integrity of any part or has turned the machine into an immobile block of rust. 

- Paint and chrome/nickel: Irrelevant; a well conserved paint is appreciated, but I can accept an exemplar that has scuffs, lost it's decals, parts of the paint, etc.

- Bendings and dents: Acceptable as long they are not extended, critical or compromise the intended functionality of the machine.

- Cracks, fissures, and fractures: These are unacceptable in the frame or any other critical part, otherwise, some small cracks might be accepted.

- Draw band: Irrelevant, however, the retainers, or hooks, of it would be appreciated. But a damaged drawband, or even missing, is also accepted. The mainspring, however, must be functional.

- Escapement mechanism: Complete; exemplars with an escapement that has broken gears or missing parts will not be accepted. Exemplars with not working escapements due dirt or gunk, as long their integrity remains intact, can be accepted. 

- Carriage Width: Nothing larger than 14 inches (120 characters). 

- Scales condition: Irrelevant; intact, missing, damaged, any is accepted, as long the frames that hold them are not missing or critically damaged. 

- Platen and rollers condition: Functional; hardening and cracks are accepted as long they do not compromise the intended

Pretty much what Uwe said, but also adding some details about my experience with electrolysis. I have used electrolysis to remove rust from the panels of an old Underwood SS, among some other large pieces of metal, personally, I would not recommend it if it is your first time with electrolysis or don't have the proper tools or implements. Here some considerations:

* The paint will be ruined, so be ready to do a full paint job. This by itself is a problem if you want to respect the original aesthetic of the machine according to it's age. The old black shinny ones and the wrinkled ones are the hardest, specially the later; the late 50's, and later plain paint jobs are easier to replicate/restore. 

* There will be a nasty goo all over the metal. Nothing a brush and some cleaning agent can't fix, but it will be messy.  

* There will be no paint and no filth. The bare metal will be as clean as it never has been, let's consider it is a complex mechanism (think in the segment and all the inner levers that make the machine work. Though the idea sounds fantastic, it also is a problem that leads to the old, and well known, question of "should I oil my typewriter?" You see, as clean it could be, it is also VERY vulnerable to rust and it will rust, fast. Personally I would use a really light lubricant for the sake of protection to the inner mechanisms, but consider this also might lead to the problem of catching dust and eventually jamming the machine. If you know the right oil or the proper way to avoid a quick rusting and/or the accumulation of dust, go ahead.  

* Is not only steel and iron. Nickel, copper and tin can also be found in the standard Underwood (at least in my experience). Consider that the nickel plating can be damaged and the chemical byproducts of the electrolysis can be a hazardous by themselves. This also brings back the matter of the paint, have you considered it's chemical composition?, the paint of back then is pretty much hazardous compared to o

I was also considering also an IBM model C if I cant find the model D, then I would consider the Olympia, and then an Adler. Thank you for the advice about the Olympia, I will have it pressent if I ever come across with the opportunity to acquire one.

Thank you for your insight Tom, it is most useful and appreciated. If the opportunity ever comes, I will try to get a working IBM Model D, even tho it sounds like a compromise due it's rarity, it also seems to be more easygoing than a Selectric. Of course, a plan for the future. 

Meanwhile I will be reading the AMES manuals of the 70's, for both standards and electrics. It is so nice to find the specific tools required by typewriter brand, handy without a doubt.