I also have plans to make an ISO-RPG one day.
Option 1:
Here's a good thread to get started with the theory:
http://forum.thegamecreators.com/?m=forum_view&t=194515&b=7
Option 2:
You can make an iso map with 3D (IRONY: Building a 2D game that mimics 3D using actual 3D).
Option 3:
Quote: "Preferably a GUI interface or a nice easy code base"
With regard to this and point 2, there are a number of engines. This one is a 2D one and limited but has GUI-based pathfinding, 2D Vector collision and everything in the map editor (including ISO-mode). You literally get a still background and "Draw" stuff onto it. The only problem is the engine was a little unstable last I checked and the scripting language is not very robust so you may need to make your own plugins in another language. However, I messed about with it some time ago and if you aren't into too much coding and you're good at using what's available (albeit little), then check this link:
http://rpgtoolkit.net/software/features/
It seems some work's been done on the software since I last toyed with it so it may be more stable now (maybe even more features). Although RPG Toolkit's aimed at making Final Fantasy style RPGs you can make loads of other games with it too, including action adventures, etc.
Conclusion:
Ultimately though, in terms of stability, flexibility and ease-of-use, you can't beat DBPro. It just takes some good ol' elbow grease to get the task done though.
Sorry about the loooong post, hopefully some of this is useful. Good luck!
