As to why LEGO won't ever release German tanks, they have had a "no military" policy since pretty much day 1. Megablocks and other clone brick companies happily do military kits and have no competition. Doubt you'll find many LEGO designer files.
Try
http://ldraw.org as a starting place for find a whole community of custom builders - it's an open standard for virtual LEGO parts for 3D modelling. Way back in 2002 I ended up volunteering with LDraw at LEGOWorld in Zwolle, NL. See
http://www.ldraw.org/community/events/legoworld2002/ for more info (me in the 3rd pic on right with grey hoodie, and 4th pic with highly fashionable bright yellow LDraw t-shirt!)
A lot of the LDraw users make their files available to download, you can then pick apart the models to figure out how to build them. Check the LDraw forums. All tryu
The only component part I ever made was this one:
http://www.brickshelf.com/cgi-bin/gallery.cgi?f=4311 but I don't think it's included in the official parts releases as the parts licensing was overhauled years ago and the email address I used was no longer accessible and so I couldn't reply to a GPL request from that email address ...
Only ever made one custom model, a TIE Interceptor:
http://www.brickshelf.com/cgi-bin/gallery.cgi?f=2742 After designing in MLCad, I then sourced the parts to build the actual model. This was years before LEGO released a minifig scale TIE Interceptor (at the time the only TIE-I was the Ultimate Collector's Series large scale model).
Also check out Brickshelf.com for loads of tank models, for example:
http://www.brickshelf.com/cgi-bin/gallery.cgi?f=318466