Hi Emrebel,
This is going to be in detail so please bear with me.
1. From my understanding your friend needs to go for Tower design(ATX, mATX) not rack mounted (link below Ref 1, Ref 2), rack mounted setup wont have enough space to host commercial off the self gpus.
2. I would suggest your friend to forget server motherboards and go for workstation bards, they are as durable as server boards and has the same set of features.
3. AI (Finally seeing the proper term hehehe) programming has moved from CPU dependent to GPU dependent a long time ago(about 10 years) so the effect of CPUs is almost non-existent(Ref List item 4 for other option). If all that your friend is going to be doing is AI, then even a mid range i7 or i5 will do.
3.1 If your friend is going to run multiple cpu heavy workloads then its completely different matter, then we are into server cpu territory. These days people will recommend server hardware the moment we say process heavy workload without really understanding what those workloads are. So I request you to please conform with your friend what he/she has in his/her mind. It will help a lot in choosing a good setup.
3.2 In general simulation workload(solid works among others) don't normally scale well past 8 cores 2 to 4 big improvement, 4 to 6 mid to high improvement, 6 to 8 ok level of improvement, beyond 8 it just not worth it. These days most of the work is been thrown into gpu as they have multiple cores and good parallel processing capability(Ref Link 3).
4. The effective role of CPU these days is mostly about feeding the data to the GPU, result verification and scheduling among other things. Said so there are instances for example Python based image augmentation where CPU power matters but there are workarounds and to say the truth, difference between a high end E5-v4 and a mid-range i5 will be a couple of minutes in total on a few hour job.
5. Regarding Epyc & Threadripper they are good processors(they will puch Xeon E and i9 and even a E5-v4 into a pulp in multi-threaded workload) but w.r.t AI, they don't offer anything that a moderate i7 or i9 can offer.
6. Server maintenance is a fancy sales term for looking at logs, if money is not an issue then its ok, in my opinion they are just money sink. The reason is most of the maintenance are easy to perform and it helps in understanding the setup better. ***Personal Experience*** I had a 250GB HDD which had 2 uncorrected sectors(bad blocks), From the S.M.A.R.T test I found out that the bad sector formed at power on hour <1320 (normal desktop disk) and has not grown, If I am to outsource the maintenance then that hdd would have been junked instead to repurposed for non critical task. ****END****
7. In the link only Z270-ws is LGA1151 board rest are different and from my observation Z270-WS is good but I believe better options exist Supermicro X11SCA/X11SCA-F/X11SCA-W (link Ref 4)(***disclaimer*** I like Supermicro boards so I always recommend them ***END***)
Ref Link 1:
https://store.hp.com/in-en/default/hp-z4-g4-workstation-4wq56pa.html (this is not a good system just for reference)
Ref Link 2:
https://www.supermicro.com/en/products/system/1U/5019/SYS-5019P-M.cfm
Ref Link 3:
https://wiki.tuflow.com/index.php?title=Hardware_Benchmarking_Topic_HPC_on_CPU_vs_GPU one of many examples, if you need more please let me know.
Ref Link 4:
https://www.supermicro.com/en/products/motherboard/X11SCA-W
I know I have not provided a clear solution as few more details are needed, please post back and I will do what I can.
Regards
B.Life