Me and some friends are making a sandbox zombie game with realistic physics, and the ability to go into any building you choose, but we just can’t get the terrain right. The terrain we need is about five island that you can get to by boat and plane, and on the biggest island is a giant city with many skyscrapers that you can go in. On every island there is a military base with a runway. All the vehicles are being delt with by other modelers so the only thing we need is terrain but that is subject to change. (You will get a percentage of the profit if the game is finished.)
Modelling a large terrain is something which is better left with algorithms and procedural generation. Especially if its going to be 5 islands worth of terrain, so I would be looking into loading in satellite data or using procedural terrain generation software, like world machine or L3dT (there are plenty more, these are just the ones I’ve used in the past). These allow you to use algorithms to generate terrain much quicker and usually with better results than going around and hand modeling a terrain. I don’t think anyone would do that for large terrain. These applications allow for weather erosion, coastal erosion, thermal erosion, glacier hollowing and things that look great when applied to large areas. They also allow you to export heightmaps, and tiled heightmaps which maps it incredibly simple for using in blender.
So have a look at these packages, try them and then decide if you need someone to model your terrain.