It might be OK for a first try, but it is definitely not realistic (the mountains particularly).
I have a scenario of Bulgaria on my old laptop, but the motherboard burned out and can't really get it off the HD.
I made it using a height map. There was a very good tutorial about making height maps, but I can't find it for some reason (again - it's bookmarked on my old laptop).
What I did was to make the height map and then use a relief map with the towns and cities on, to put them on the map. I basically look up a town on the relief map, then look for the same geographical features on the map window in the Scenario Editor. For Bulgaria it works fine, because of all the mountains. It's not the most accurate thing in the world, but I think it's quite sufficient for TTD.
I tried making rivers, but it failed miserably. For it to look nicely, you need the correct river beds. To do that you must create a very big map. To fill that map up with towns, industries and everything it will take too much time for me.
A note about the GRF's:
For the train set, use the CD Train Set, as most of the electric locomotives are Skodas. You can also use the Romanian Train set (for the locomotives class 46), but last I checked it was still in an early stage. The diesel class 07 can be found in the DB set (I think it was the DB 232).
And for the vehicles I would suggest the German Road Vehicle Set http://grfcrawler.tt-forums.net/details.php?do=details&id=77
as there you will find the Ikarus buses.