Gonna be honest, didn't read the whole thread, but there's a few thing you should be aware of.
I was set on making a small demo RPG for one of my game dev classes last year. What I ended up finding out is that in order to tweak Unreal Engine away from it's FPS/Deathmatch oriented gameplay, it's going to take some coding under the hood (along with some good kismet work).
For example, just trying to get enemies to spawn at trigger points AND be intelligent was damn near impossible for me. Either they existed when the game was created and had intelligence, or spawned in at the trigger point and just stood there and shot (no running). Also, custom skins on Unreal models, at least before the UDK, was difficult enough for somoene new at it to just abandon it for the time being.
My advice would be against UDK for RPG's if it's mostly foreign territory for you. It would be a shame to spend most of your time learning UDK instead of making your game.
Unity, Torque and XNA are all very viable options. Unity is free now too! And XNA has always been free, but you have to pay $100 if you want to develop on Xbox. If not, you're fine. No strings attached as long as you don't use MS's code or media in your final product.
EDIT:
Not sure if you can bring your terrain in. UDK uses subtractive editing while DBP used Additive. With unreal, you're essential scooping out and hollowing a massive solid cube that your map is in. DBP it's like your in outer space and you're just adding stuff in.
Unreal is really easy to learn so long as your doing things it naturally does. There are EXCELLENT free online video tutorials you can follow that will have you making pro death match levels in a days worth of watching. I own both the UDK books currently, and they are excellent as well. I don't think the third book has come out yet.
"If I have seen a little further it is by standing on the shoulders of Giants" - Isaac Newton
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