I have not yet installed Deled editor, but I have hopes on it. Specially as supports x, and that's a format becoming widely supported by engines.
In general, for any engine (Irrlitch, DbPro, etc) I'd put number one priority that what you can't make with other available free tools. And that is lightmapping. So, first I suppose come lights. Some basic lighting (dunno if radiosity is more effort cost than standard lighting, sounds that yes) and the actual generation of the baked light/shadows tga (lightmap). And ability, support I mean, of uv channel 2. As you need it to assign to this second uv coords set, the tga, the actual lightmap.(while standard texture uvs are in old uv channel 1 ) Ultimate Unwrap supports this nicely, you can have a look at the only save crippled demo. But ultimate can't generate lights, nor lightmaps.
Indeed, the only tools for free I found, I could not make them work.
I would not worry that much about producing a BSP, it is too unclear if legally bsp format can be used legally. Some people say yes, som not. If you use routines from ID, then sure is not legal for comercial projects. If you use halflife BSP (there's already a free hl bsp compiler, the ZHLT tools) that's free. Even more you could add a map format (maybe map?) or whatever eats ZHLT tools.
But... IMHO is more uptodate, flexible and powerful just to use *.x format ability to use two uv channels, one for lightmaps. I'd priorize that.
Modelling can be done outside for free (Wings3d, Metasequoia, OpenFx, Anim8or, just to mention a few) .Uv mapping can be done with Wings, some yet keep the free copy of Lithunwrap, and Uvmapper classic(called beta too) is yet available. You can import a very complex mesh, usually much more complex than even very old and famous level editor can do. I worked in a company and trend preferred by many artist is work in his tool (Max, Maya.. ) and have a mel or maxscript export towards engine or editor. Big packages generate radiosity wonderful lightmaps, but go and search a lightmap generator totally free (comercial projects allowed, etc) You don't find it. Fsrad could be but you need to be a coder to adapt it, and the ase import(ase also is not common format, I just have Ultimate Unwrap so I could export one) did not work for me...(I'm an artist)
Second in priority after lightmaps, for me it would be export import plugins. But mainly...halflife BSPs (for the other legal issues).But don't invent the wheel
; There's already a free hl BSP compiler for free, you just have to download ZHLT tools and see which is the format (surely the initial of Warcraft/Hammer, maybe called *.map.Ask some hl modder...) But I don't know how well this format support lightmaps and other features; don't know if it is as good as less free quake3 bsp format. Still, if I remember well, HL and CS had nice lightmaps, so...
Anyway, BSPs(old format) has its problems for large terrains. Once again, for me the important would the more than standard in packages and engines x file with 2 uv channel support for lightmaps (I don't know how hard would be for you to generate lighting (initially could be just a lighting that generate smooth border and transparent shadows, if radiosity is too hard.))
And I think that lightmap stuff is already quite a task...or maybe not. Maybe is easy. I am not a programmer.
