[Looking for mods that add dimensions.]

Yes it is but the end is out. you have to kill the dragon to build it without it's destroyed by the dragon. or just search some mods on google, MinecraftForums or on youtube the best way on youtube is AdamZoneTopMarks that is GameChap.

[Number of Maps]

and in that case of maps you have to use commands to make other Map ID's.

type in /give 358 {the id of the map} {how much you want}

[Number of Maps]

Yeah offcourse there's a limit if you've got to much save in your saves folder, Technic Will Crash even minecraft does by to much saves.

[NEI not showing up at all?]

you need to delete some files & folders: mods, Bin, Resources, options.txt, Config, stats, redpower, Matmos(all) and Buildcraft and BetterDungeons. than re-open technicLauncher and download the Pack again.

P.S do not delete your saves or texturepacks otherways you can't restore them.

if it worked Your Welcome