Apparently there are some ship editors out there, but I've never tried them (I think one of them is called "The Docks" or something) as they are PPC native.
I think the people here have offered some very good tips, but I'll add my own opinion to this.
Personally, I'd work with EV or EVO Developer's Map for the systems, Mission Computer for the missions, and ResEdit for everything else. If you think you can use EV-Edit, go ahead; apparently some people have been fine with it. However, in my experience, I've had far more trouble with it than with learning ResEdit, and it's not as hard to learn as it seems.
I'd sit down and take it one step at a time. Take the first resource you want to edit; say, the governments. Start your plug-in file, keeping backups along the way, copy the templates in, and start playing around with the fields. Often the best way to learn ResEdit is to make a copy of EV and mess around with that first.
Basically, if you feel inclined to learn ResEdit, you'll have less problems in the long run, but I guess it is a big task to learn in the first place. In any case, I wish you luck, and if you have any more questions, don't hesitate to ask here.
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Mike Lee (Firebird)