This question has been booted around in forums a lot, and there are many opinions. This is my two baht worth:
Much depends on how realistic of an instrument you wish to create.
If you are experienced with XML and want to make a 'FlightPlan' Management Computer, even that is a complex task. Take a look at the default gps500.xml gauge. Although it's not exactly an FMC, you'll get some idea of what you will be in for.
If you really want to make a proper FMC, that's very difficult and best tackled using C++. Ed Wilson
said it best.
Bob