First of all, avsim.net and flightsim.com are the two best sites out there for downloading add-ons. They both require you to register, but it is completely free. The risks are minimal, and I have never had a problem in the years I have been downloading add-ons off the net.
It will take up hard drive space, but not too much. Right now, I have a good deal of add-ons, such as scenery and aircraft, and my FS9 folder is at about 5 GB
. And that is after about a year and a half of downloading add-ons.
As for easiness, it is very simple, especially once you do it once or twice, since most aircraft add-ons simply require you to drop the folder in the aircraft folder, and you're set. Here two more threads on this topic that should help you, along with links to avsim and flightsim:
FS 2004 Newbie (by Hawk44 Apr 10 2005 in Aviation Hobby)
Importing New Aircraft (by Hawk44 Apr 16 2005 in Aviation Hobby)
BTW Just another thing, if you buy any payware, make sure to get Utimate Traffic. It is amazing, and makes the simming experience that much more realistic. If you don't want to spend the money, Project AI is good, but not nearly as good as Ultimate Traffic.
[Edited 2005-04-18 02:19:12]