Hexagon will save an STL file, although in text format only. Binary STL format is more common now, and makes smaller files that are faster to open/save. Its possible to use Hexagon for 3d printing. I have done many projects in Hexagon that have been 3d printed, in conjunction with Netfabb. The biggest issue I have with Hexagon and 3d printing is that its a 32-bit application, and even using LAA to allow it to use up to 3Gb of ram (without LAA it will use up to 2Gb) it can barely handle 1 million triangles. Depending on your needs that might be enough, but I have found for detailed relief or sculptural work, at least 2 - 3 million triangles is a must. 

Its fairly easy to end up with errors that will be an issue for 3d printing regardless of what modeling software you use., although some of the processing software used by the printer seems to be more forgiving than others.

I used the free version of Netfabb for a while to fix errors and it was pretty good, and then purchased the Pro version. The Pro version will also repair intersecting triangles. In my experience Netfabb is least expensive, most capable tool for repair. The price went up recently, I think it is now in the $1500 - $2000 range, but it is still far less expensive than Materialise's Magics (which is about $8000). Netfabb does have a less expensive "home" version, but I have not used it so I don't know how it compares in functionality.

There is a program called Leveller http://www.daylongraphics.com/products/leveller.php which may be useful if you are producing reliefs, and its only $50.  Its marketed as a terrain generation program, but it can produce reliefs by displacement painting, heightmaps, and meshes created in other applications. Some advantages is are you can't make intersecting triangles, or undercuts because of the way it works. Also, it can easily handle many millions of triangles. The biggest limitation from a 3d modeling standpoint is there can only be one Z value at a given XY location. So you can make a half sphere, but not a full sphere, for example.