Unity and UDK for Mapping

I plan on making a game that I could possibly sell using unity. But one of the main features I want is the ability for players to make custom maps. Since UDK has a huge amount of people that know how to map in it is it possible to release a unity game for sale and include a little tutorial of the pipeline for making maps in UDK and getting them into my game? Both are free to use so I don’t think there would be legal problems. I also would need the servers able to send the custom maps to players when they connect. Is that possible? Finally if UDK is a no, then what alternatives can any of you suggest to make mapping as easy as possible for my game by the community. Thanks

“is it possible to release a unity game for sale and include a little tutorial of the pipeline for making maps in UDK and getting them into my game?”

Sure.

“Both are free to use so I don’t think there would be legal problems.”

Correct.

“I also would need the servers able to send the custom maps to players when they connect. Is that possible?”

Yep. Unity can load data from servers using the WWW class or .NET sockets. It’s up to you to set up the server and figure out how to implement it.

If you’re looking for alternatives for creating maps, there are a number of good ways to do it. Perhaps start with WorldMachine, which has a free version.