While I am, admittedly, a great fan of the Abacus products ..  the first question is what version of FS are you planning to build for?

For FS9 ... you really don't need to spend any more money.

-gmax 1.2 comes in CD-1 (Extras) and
MS has recently released the necessary gamepack modules - converters and plugins - necessary to create the basic model, virtual cockpit and animations.

- there are some freeware programs to create the 2D cockpit/panel (and at the very basic level, you can really define the 2D panel with Notepad.)

- You will need a paint program (even MS Paint will do, in a pinch), and even imagetool, which converts from the basic bitmaps to the eye-candy formats recognized by FS - comes with the Microsoft SDKs recently released.

Having said that,  the Abacus bundle is a good one. 

FSDS is a good airplane modelling program, with which you can build and export a model into FS9 with few, if any, problems.  The overall interface, *in my opinion* is easier to navigate and get a flying model than the gmax semi-manual process.

VASS - is a paint editor that allows you to assign and manipulate textures to your FSDS source model.

Custom Panel Designer, lets you create the 2D panels, although it does not - at this time, that I'm aware - let's you actually create new gauges.

In any event, reading the Microsoft SDKs for the version you're modelling is imperative!!