Do you know if the code base is in Fortran 77, Fortran 90, or something more recent? It doesn't make sense to look at code base which uses a lot of Fortran 2008 features if you are going to be using F90.
Which field will you be working in? If you are going to work on fluid dynamics it may not make much sense to look at an astrodynamics package.
My limited search found a few places you might look at where people asked something similar to your question:
https://stackoverflow.com/questions/2758635/writing-robust-a...
https://scicomp.stackexchange.com/questions/2328/are-there-o...
As I said, I can't answer this topic either. I did find https://scicomp.stackexchange.com/questions/8319/learning-fo... which has more suggestions about resources, including http://www.fortran90.org/ . There's also https://stackoverflow.com/questions/2758635/writing-robust-a... .
The author of the new book "Modern Fortran: Building Efficient Parallel Applications" is on HN, and posted about it at https://news.ycombinator.com/item?id=16700789 . The discount code has since expired.