I voted for modling. Mainly because I can't and without models (or people that can work on them) your mod is basicly screwed before it begins! Substitues can work if your desperate but in the end you NEED models and MODLERS. No ifs, ands, or buts!
In second place I'd say getting turrets to work CORRECTLY as well as placing them (but I'm only guessign from the posts people make about it)
Third problem but also a biggie is mapping, (as once it's mapped you can spend how ever long it takes to find a texture look that works) but you cant experiment till it's mapped! It took me about 4-5 hours to get a decent texture scheme for Lar's model that I use as a generic White Base (Gundam) due to his raw model already being mapped! (thanks again to Lars for that!)
[EDIT] I can see Tin Can put much thought into his post
