Argio wrote:

How does the Model D compares to the Selectric in these aspects? Is built to last...

 
When it comes to standard electric typewriters I'd suggest that they were all built to last. Some models may have done better than others in that regard, but given they all were designed to be used in a professional environment, and were expected to endure the rigours of such use, they had to be durable or the company (IBM in this case) would have had a difficult time selling subsequent models to the same customer.

Argio wrote:

...is it also known for requiring frequent attention, adjustments or replacement of parts?

Tom (thetypewriterman) is likely the only person here who could provide the first-hand knowledge required to answer that question. Current collectors and those who service machines today wouldn't have the necessary exposure to enough Model Ds to answer the question. In other words, you would need to ask someone who used to repair such machines during their service lifetime.

Argio wrote:

...in case a replacement (part) is needed, is it a procedure complex enough that would have to resort to professional help?

It would depend on the part in question. Like all typewriters, some parts are more easily replaced than others, and if the easier-to-replace parts are the ones that are more likely to break, then professional help wouldn't be required as often. 

My guess is that if you're proficient in working on a standard electric models from other manufacturers, the Model D would fall into your capabilitities. One thing I've learned about repairing typewriters is that each manufacturer had a long list of specialty tools designed for working on its machines. If you had (have) those tools your life is much easier. Without them an otherwise simple repair can become very complex. IBM was no different in this regard.
 

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.
 

Those machines look sturdy, I would take only a minimised controlled risk, being with someone that could transport it safely or a seller that could provide a safe shipping (I am aware it could be a bit expensive), but this only if I buy online. 

I have seen some old IBM model C machines laying around at my faculty, I would not be surprised to find a Model D if I ask in the right places or to the right people.  

I must agree about the recommendations, specially the Olympia in my case (but that is just because I really like the SG3 aesthetic).  Also, it is somewhat a relief that your experience repairing your machines was straightforward. Definitively I will consider the Olympia SGE50 and the mentioned Adlers as immediate options, just in case acquiring an IBM becomes way too hard.
 

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 the article, proportional spacing sounds like a very interesting feature for a typewriter, speaking for the executive ones of course. Reading a good opinion about the touch and other features is also helpful.