You will always be able to play online but here's the thing, once you install a single mod (doesn't matter which one) you'll be connected to seperate servers to keep you seperated from the people that have unmodded games. So even if you only have a graphics mod you'll still end up on servers where 9/10 people use godmode cheats and the like.
18.104.22.168. is fine too, most icenhancers are indeed made for that one. If you want a good icenhancer just go to www.gta4-mods.com/misc
and sort by most downloaded.
As for installation, graphics mods are usually a drag and drop kind of install so it's really easy. Just make sure you have your game backed up in case something goes wrong or in case you don't like the graphics mod.