It really depends on what game/engine you want to build levels for but I could make a suggestion...
You could try downloading the Skyrim Creation Kit from Steam. This is really only useful if you have the game, of course. You will find plenty of info on YouTube regarding how to use it.
Torchlight 2 will also feature a level editor but is yet to be released.
As for the 3D software, try http://www.blender.org/
- it's free to use non-commercially.
Another free 3D app that you may find useful and fun is http://www.pixologic.com/sculptris/
- Check it out.
Your proposed system should run these programs pretty well.
I hope this helps and good luck with your hobby.