from my experience on this, 18's are good for the golf mk4 but can be a bit harsh on standard suspension and not to everyone liking. 17's seem to give the best handling and comfort and then 19's look the nuts but ruin the way the car rides.
At the end of the day its all down to personal preference, where I find the 18's acceptable someone else may think they are too harsh and so on. Depends on the condition of your roads as well. As for fitting them, no there should be no problems with the standard suspension, if your lowering the car, once you start getting to the 40/50mm stage you may need spacers.
