First of all, is a Mac Pro really worth the money?
If you find a good second-hand model, then yes.
New, not at all. The Mac Pro is still from 2010, that means no USB3, no TB, no SATA-III. no Sandy/Ivy Bridge and so on.
Secondly, what makes the Mac Pro Server different from the Quad-Core and 12-Core?
Mac Pro Server? Never heard of that. I know there have been G5 servers, but I don't know a Mac Pro server.
The Mac Pro uses server technology anyway, such as Xeon CPUs and ECC RAM.
Thirdly, How much of a difference would getting a SSD affect my rendor speed in Cinema 4D?
Pure render speed, nothing. That's just CPU, nothing else.
But a SSD is definately something that has to be included in every high-end machine. You will feel your machine is 10x faster. Faster Boot (under 10s in a RAID0), apps open faster (I don't even see the C4D loading screen, it opens directly in just 2-4 seconds) and everything.
And lastly, would I get a cheaper Mac Pro if I buy the RAM, HDD, SSD, and Graphic Card separately and not through Apple? Do they actually sell these items that are compatible with the Mac Pro? If so, can I get links of these items?
Do not ever buy that stuff from Apple. Everything of what you mentioned costs about twice as much.
A Mac Pro uses standart components - it's actually easy to find them.
Be sure to buy ECC RAM, this is very imprtant. Otherwise, the CPU won't be able to use the RAM.
In terms of GFX cards - here on the forum you'll find several threads of how to put a officially unsupported card into a Mac Pro.
I did put in a GTX 570 in mine, and I love it.
Read these 3 threads:
https://forums.macrumors.com/threads/1440150/
https://forums.macrumors.com/threads/1360927/
https://forums.macrumors.com/threads/1385329/
These are also useful:
https://forums.macrumors.com/threads/1280517/
https://forums.macrumors.com/threads/1122551/
(In the Mac Pro, you can update the CPU yourself. Careful, you'll loose warranty.)
Cheers.