Hi tlmaen,
Here's some answers, there are others here who will be able to answer the other questions much better than I can:
First, to a question you did not ask

the SL4000 is very easy to work on. They were built by hand, and everything can be repaired by hand. A key feature of the design was that if it went wrong it had to be VERY fast to get working again.
One exception: personally I would not have plasma bargraphs, they are HORRIBLE to work on. It was difficult enough to get them working when they were new. If you do buy this one with plasmas make sure you get LOTS of spares. Or allow for the cost of replacing them with VUs.
- What functions do exclusively rely on a mix computer (Of course all Automation and TR)? But are the VCA groups are working without a computer? Transport buttons?
Yes, VCA groups and transport buttons work without the mix computer. As you say, it is only automation and TR which needs it.
- Is it possible to modify the Channels to implement Solo Isolate?
Yes, the solo function is very simple on the 4000.
- How is the Sound of the Micpres (They have Transformers) compared to the MPs without Transformers?
You'll need to take other advice on this, and perhaps listen to them yourself. Of course this can be very personal. But remember the transformer in the transformer mic pre is very small (it had to be to fit in the channel strip) so is not great at low frequencies.
- Is there anything special compared to the later, post 83 E Series?
As you mentioned, the tantalum capacitors, which may actually be in better condition now than a later console with electrolytics.
The dynamics sound different. You know the switch on the 9000 which switched the dynamics between two different responses? That was switching between the early 4000 design and the later one.
The VCAs sound much nicer in older desks! Sadly DBX discontinued the original design around 1985/6.
The routing switches I think changed manufacturer? Only important as some will probably need replacing, so you need to know you can find modern ones that will fit.
Probably other things I've forgotten, Somewhere on this site Matt (with a bit of help from me) compiled a nice history of the changes, but I can't find it now? But I can't think of much else that changed until 1986 when I changed the earthing arrangement (which only matters if it is a 64channel or bigger desk) and 1987 with the G series which gave lower distortion.
You can fit any of the later channel cards into a 1980 channel strip, although you will also need to change some pots for later EQs to work to their best.
Obviously I would always recommend everyone to get a 4000

(even though I did more design work on the 9000, the 4000 is much easier to keep going - as you say, no SMD!) But yes definitely worth taking advice from people here on what to look out for on a very old desk!
Cheers, and good luck - we'll be with you to help on your journey!
Andy