Or what version of M:C should I use this mod ?
M:C is still undergoing heavy (and interesting) development / changes.
Building a modmod from a mod that is still changing so heavily, is not necessarily easy, since a lot of new features and functionalities with XML changes (units, professions, ...)
and bugfixes or improvments as well are likely to be developed (or changed) which will need
continuous adjustments in the modmod.
Most of the things that happen in DLL considering new features affect XML.
So even if you always simply use the DLL of
M:C, you will have lots of work.
If you
branch your modmod
now, you will have
2 alternatives:
1) Constantly adjusting to new versions of the base mod
or
2) Missing a lot of interesting improvments and features
Both alternatives are not really great.
So if you want to build your mod on
M:C most likely the best strategies would be:
A) For
now try to
support the base mod so it will progress faster.
(At some point
in the future, use a much more advanced and stable version of
M:C to create your modmod.)
and/or
B) While
M:C is still under heavy development use the time for
general preparations.
(Creating and discussing concepts or colllecting graphics, sounds, background information, ...)
Or is RaR maybe better platform for this mod ? M:C of course has the tech tree implemented.
RaR is more "mature" but also still constantly undergoing changes and enhancments.
(Changes / development is not as heavy anymore as it still is in
M:C.)
But considering a mod(mod) for
modern times,
M:C does have some
better suiting features and concepts.
(e.g. Land Starts and Technologies)
Things also look very differently for programmers and non-programmers.
As a non-programmer you might simply not be able to change
RaR yourself into a mod that really suits "modern times" in a way you like.
(
RaR features simply do not try to suite for "modern times" and never will.)
As a programmer, adapting the "modern times" features from
M:C to
RaR is of course possible.
But I just don't know how many
M:C features you would want to adapt and thus cannot estimate how much effort it would be.
--------
Summary:
The only person that can really know how you want to proceed is
you.

Every path you might chooses has its advantages or disadvantages.
I believe:
For your plans
M:C does fit much better considering concepts and features.
However, it is currently still too early to seriously start working on a modmod based on / branched of
M:C
because the base mod is still changing too heavily and still has several issues to solve (e.g. Multiplayer compatibility).
Currently it would be much better to
support M:C base mod development so it might progress faster and branch of your modmod some time in the future.