Two words: Real life. I hate it when RL interferes with Skyrim!
Glancing at the scripts on scripted Enc dragons, one small script (MG05RitualDragonScript) just allows the harvest of dragon heart scales for the Alternation master quest. It's not necessary, but if you leave it out, you will probably have to explain why players can't harvest the heart scales from YOUR dragon.
The other, much longer script, deals with all the little details of dragons, from what skin they use to setting up NPC interactions (for ex, when you kill a dragon near a town and people come running to gawk at it). It also covers crash landings, perching behavior in towns, absorbing dragon souls, etc. I can't advise you on this one, because I don't know what kind of behavior you want for your dragon. The only thing I can say is that it wouldn't hurt to copy the script body into a new script, and then edit out things you don't want. Be sure to check the script properties and makes sure they are what you need, and don't leave anything extra in there. Just as long as you don't change the original script.
(Scripting is what makes me

and

so ask someone else for answers on that!!)
For the dragon alias, if you
want a one-of-a-kind dragon, it just makes sense to
make a one-of-a-kind dragon. You can use the Template Data thingy at the bottom left of the new Actor's window and base it off of any encounter dragon you choose, so much of the work is done for you.