I started off learning 3D Studio Max in high school as a part of vocational training in multimedia graphic design. But since the teacher was primarily a video editing guy the three of us 3d artists had to figure things out on our own. Pretty cool deal, considering I only had to take 1 math and 1 english course my junior and senior year and spent the rest of the school day doing this stuff!
3D packages I've worked with since: AutoCad (Civil 3D), geomagic, lightwave, rhino, blender, softimage XSI, and of course Maya.
Sculpting programs: ZBrush and Mud Box
Other Misc 3D: Unreal Editor, Vue, Terragen, Cyclone
In college, Maya was the standard that most of the courses revolved around. So naturally that became the one I invested the most time with and became the quickest in (modeling-wise). After working with different programs I started to see all of the 3D package as 'toolboxes' rather than software. And for every toolbox you pick up, all of the tools are usually the same they are just in different places in the box... and sometimes called something completely different, ha!
My point is when you are strictly talking about 3D modeling you are talking about the very base level concepts of 3D work. Everything starts with your object. And every surface type that you use to make your object gets broken down and tessellated into polygons when you render so I am just going to say that polygon 3d objects are the first step, and the most basic parts to 3d work. Now, what toolbox (program) you want to use to build them is entirely up to you and usually is based on which one you spend the most time in. In my case I'd consider myself a Maya user.
When you want to take a 3D model and use it for things other than Pepakura then pros and cons between software packages become more relevant.
AutoCad and Softworks are very good for product design that can be used in Prototyping or CNC. Programs like those are very good for getting exact scale and surface measurements. But I personally do not find them suitable for Pepakura, Foam, or builds that require you to flatten and print because those objects tend to be made using curves with a really high tessellation.
Maya, Blender, Softimage, rhino, and 3DS max are good for keeping your model simple geometry-wise and are meant to be able to edit and sub-divide quickly. They are great for building using image planes as reference and give you a good sense of control. And because you aren't typically using curves to generate these models it allows you to keep your tessellation (resolution) low and close meshes more efficiently.
Not that you can't do this kind of work out of Solidworks or Autocad, you can. But in my experience the process and tool selection felt limited and bogged down. People that typically know product design know how to work wonders with those softwares.
I do 3D work on both Mac and PC (at home and at work) and I have no trouble doing my work with either. When it comes to rendering I may sometimes opt to go with Mac for other reasons, but for the most part I say use whatever you got.
Here is the link to Zakuace's thread:
http://www.405th.com/showthread.php/26508-Halo-3-ODST-Visors?highlight=zakuace
As for my actual 3D process. I usually get my reference together first and try to get the front/side/top views aligned as evenly as possible in photoshop and adjust the levels to see detail in dark areas. I always use a 10x10 in canvas at 150 resolution and save the views out as their own separate jpeg files. The images seem to come in smoothly into Maya as image planes.
I almost always start with just a simple plane. My process basically involves creating "Patches" of the model and slowly bringing the elements together, modeling one section at a time as opposed to making a huge low poly object and constantly sub-dividing/editing it. Sometimes though, I'll feel like the objects will work better as separate pieces for Pepakura and leave them that way.
I then set my grid up in Maya accordingly. I actually have a 3D mannequin version of myself that I created a while back. I took measurements of my own body using a set of calipers and scaled my 3D model to fit into those measurements in Maya. This process isn't that complex, but it took time. And I only had to do it once so unless I undergo a dramatic weight change I can just reuse the scene file for other projects.
I import the 3d objects into the armature scene and scale them until they fit and look good on me. Then I just select whatever object I want to build and export it out as a wavefront .obj file.
When I bring the objects into Pepakura even though my scale is right in Maya, Pepakura views the model 10 times smaller than what it is supposed to be. So after I hit unfold I change the scaling of the object manually using a factor of 10 and set it as the default for all other imported objects.
As for modding, just be sure to get permission from the original author. More often than not with my files I won't have a problem as long as someone asks me first.
Well you have a lot of options to reduce fog in your helm. The ideas you propose will all help. The great thing with the ODST helm is that it is elongated and the Visor isn't up in your face. So you have plenty of room to work with as long as you are smart about padding.
In paintball I remember small fans being available for the masks, back when I played a lot. It was a while ago, I didn't find the fans very useful when running around. But they'd work great for conventions. They're small and their shape would probably fit in an ODST helm easily. The ones I've seen would typically be located above your forehead inside the helm.
And those vents on the helm model are optional. You could build them, and harden them, then cut out the space in between. But I decided for my build it wasn't worth the extra time when I could just have the option to cut those areas out and put something better in later.