No, SceneGenX isn't the 2k4 version of Airport. It's a handy little util to place objects into 2k4 at specific lat/long coordinates using the XML SDK (Scenery or Software Development Kit) released earlier this year by MS. It is what I said - handy - but it is in an early stage of development and currently can't do a lot of things - like place runways for example which is pretty essential in 'new' sceneries. For that you need AFCAD (which itself is very useful, but can't place objects and buildings) so one approach is to combine the 2.
But then you find that with SceneGenX you are limited to the ability to place a relatively small number of very specific default objects and vehicles. And as I found when I introduced my own MakeScene package in the early days of the SDK, many of the objects that are in the list are not actually usable as either they just do not appear or are latitude/longitude delimited (ie a JFK hangar or building can only be used at JFK or in a very small region close to the airport).
So where does this leave you? Well until some of the advanced tools such as AFCAD and SceneGenX are further developed, with the old tried and tested methods I'm afraid.
First you need a scenery design program. I prefer the latest version of Airport but used to use FSSC (Flight Sim Scenery Creator) which now has certain shortcomings in FS9. So I'd recommend if you are starting from new to go for Airport. You will have to take my word on that. I've tried to do certain 2K4 scenery things which I'm not going to talk about on the forum in both FSSC and Airport and they only worked in Airport.
Then you need objects to place. Either you can download an incredible number of many different types of macros (and there are stacks for what you want to do - WW2 buildings etc) and place them using your scenery program OR you can make your own. Then you would need to either learn GMax (which is a 4-6 week job minimum) or you use EOD (Easy Object Designer).
If you make your own objects you will need textures. Very often you can use whole or bits of default textures which are in your 2K4 Texture folder but sooner or later you will need to make your own. I use CorelDraw, PaintShopPro and PhotoShop for my graphics, plus you'll need utilities like DXTBmp.exe to convert them into the correct formats.
You can create runways, taxiways, aprons etc in Airport or in AFCAD. Personally I find the AFCAD stuff easy to do but crude looking - sorry - and there are always faults with dodgy joins and intersections. But as a beginner, it's probably the best way to start.
So there ya go - some food for thought from a scenery tinkerer