I can't make models, though alot of the people who do use anim8tor or something like that. There is probably about 20 vehicles to fly in that game
I just delve into editing the text files of the units created already to adjust them to my liking.
That Ariane 5 and Buran are two seperate units in the game, and the Ariane 5 is unique in that you can set what it's payload is. I edited the scenario file and changed it to the Buran shuttle (which currently lacks a booster rocket).
The game itself only comes with four rockets, three of which are fantasy ships. The rest are 3rd party mods.
1. The Shuttle Atlantis.
2. The Delta Flyer. Takes off vertically with hover jets, and has enough fuel on it to reach Mars. Thats the red plane you see in a screenshot up there
3. The Shuttle A. This is similar to the Delta flyer, but is bassically an non-areodynamic "space truck". It too has very long range fuel tanks and has hover jets for take off and landing vertically like a Harrier jet.
4. The PB Shuttle. This is a tiny, single seat pod. Again, lifting rockets as well as a main thrust rocket. Vertical landing and takeoff.
Now, there are a few websites that add all sorts of new vehicles. Two of my favorites are the X-33 Venture Star carrying an X-37 as a payload (both theoretical prototypes build by NASA), and the old Dynasoar 2-man shuttle on top of a Titan-3C booster.