Maps: Best method to convert Mesh to Rhino single surface

I have a map surface in a complex mesh format generated by a physics-based depositional model that I would like to convert to a simple Rhino surface. After playing with various Rhino Functions and Grasshopper scripts others have suggested for converting Mesh to Breps I cannot seem to generate useful results (I can get nice Polysurfaces by various methods, but I need surfaces easier to hand edit and modify in grasshopper). For example I need to remove overlap of different superimposed surfaces, which is fairly easy with two simple Rhino surfaces built from the same number of points but not so easy with a complex mesh.  Despite two days of effort I cannot get simple surfaces from grids that are useful. I include a simple example of one of my grids here, showing a single river channel crossing a nearly flat floodplain. Other examples have more complex surface topography, but the grids are similar in that all points are a monotonic function in the vertical (Z) direction (that is, surfaces never fold over). I do not require high precision in the generated surfaces, but they need to remain reasonably similar in look to the mesh surface (i.e., no wild artifacts). Following my  efforts, and other reading in this site, I conclude that I need to buy an add on to Rhino for this type of operation ... which falls generally under the general label of reverse engineering programs.  Some choices seem to be Resurf3d (~$500), Rhinoterrain.com (~$1500), Evolutetools-for-rhino (cost?), T-Splines ($675...maybe?)... none cheap but I would pay the price if I knew it would allow me to proceed with development of the rest of my modelling workflow. Any suggestions for the best methods to get from surface meshes to simple Rhino surfaces? I guess I could just download trial versions of each, but I am on the steep end of the learning curve on Rhino and Grasshopper as is, so am hoping for some pointers in the right direction from those with experience before I jump in. Thanks for any suggestions.

Brian Willis