Hi FILIPPO,
I'm not an expert but I'll weigh in with what I think I know...
-I think the Mag 3D/12AA setup is a good choice for a 100 watt build. Not sure if the Modamag adaptor fits in an unbored Mag. Fivemega says his does not.
-Not sure where to get the 3D/12AA adaptor. Fivemega was hinting that he might make more, but the thread hasn't been bumped for a while so who knows if/when it will happen.
http://www.candlepowerforums.com/vb/showthread.php?t=114510 (see in particular pages 5 and 6)
-I agree with other posters that you need high current NiMH AAs. I plan on getting the Titanium 1800s from Amondotech, but I'm in the USA so shipping isn't the challenge for me that it is for you. I have not read about anyone using Eneloops in a 100 watt build. Like TigerhawkT3 said, they seem to be good up to about 8 amps load, but the 100 watt bulbs will draw in the neighborhood of 10 amps.
-Bulb: The USL bulb (Osram 62138) is best with 11 cells (I think it will flash with 12 cells), so you'd use a dummy cell in a 12AA adaptor. The bulb I'm planning to try out with 12 cells is the Osram 64623. Consensus seems to be that if you want a nice beam with a very white color, the 62138 is the way to go. If you want brute power and more lumens, but a less white, more artifacty beam, use the 64623.
-I'm going with Kiu for my socket. For this high-power application it seems like it's a better option than the FM bipin adaptor.
-You'll need a reflector with a bulb hole diameter over 0.5 inch. Not sure where you'll find one of those though.
-Definitely a borofloat lens. They are available from several sources (Fivemega, flashlightlens.com, Lighthound). I would try to add it to an order for something else to save shipping (in other words, if you buy a reflector from Fivemega, get the lens from him too).
I asked a similar question a while back and got some good advice back.
http://www.candlepowerforums.com/vb/showthread.php?t=166692
I haven't built the light yet, but I'm planning to soon (I've been distracted by the AW Mag C incan driver). When I do I'll let you know how it goes.
-Rick