Estimate cloudbase height
You are right, you can't get cloudbase height without 3D data. You can do that using radiosondes. However, if your weather station has a ceilometer, then you don't actually need many calculations. However, they are usually quite expensive.

If you want a real rough calculation, you can just use the Lifted Condensation Level, which is more valid estimate during the day than at night.

Note on the grib files: They may not always have all of the variables. For example, the WAVE ocean model (see NCEP NOMADS) won't have cloud base, because it is in fact for oceans. However, what you can do is use models like the GFS, which tell you what model levels have clouds, and then find the height of the lowest level that has cloud cover. It will take some programming though.