The software packages that I would use to achieve these things would be : (keeping in mind that you asked for freeware as opposed to commercial products)
1-3 : I would suggest, for a free package, you would use possible "Blender 3D"- an open source 3d modelling package.
http://www.blender.org/
or SoftImage's XSI Mod Tool(this is a "lite" version of SoftImage's XSI commercial and industry standard product, it is on par functionally with high-end products such as 3D-Max. With SoftImage's XSI mod tool though, data transfer could be a problem as you may possibly need to create your own importer/exporter(although XSI publish a great SDK to help you achieve this)
1. A charector with removable body armor.
2. A range of weopons with scopes on some.
3. A range of vehicles.
As for creating terrain and "levels" there are a number of options availble. You could just use the above packages, and model the level and terrain as you would any thing else. However, there are also a number of specific terrain and level building applications about that are both free and low-cost. Some of the best ones to use with DarkGDK(and DarkBasic aswell) are listed on TGC's homepage and some of them even have free trials so you can try them out before you purchase them. I would reccomend trying T.Ed or 3D World Studio, both are fairly easy to use in conjunction with DarkGDK/DarkBasic
On the free side though, Terragen Classic and Terragen 2, are available as Free download(for non-commercial use) from :
http://www.planetside.co.uk/#
Also for creating Levels, you could look into some form of "BSP editor" such as "Hammer" that comes with the SourceEngine SDK or even "Nem's Tools" - a bunch of level design tools, oriented towards half-life, but easily usable with darkgdk.
4. A landscape.
5. A level.
If it ain't broke.... DONT FIX IT !!!
