Rom manager and cwm recovery are pretty out dated. Most are using twrp recovery now. As for how much space apps on external storage use on internal, depends on the app.
4 GB is a decent amount of space.
If you want to update without wiping, you can. You said you were on nd8. Just use odin
I found a page that has Odin 10 . I'm assuming that would be ok to use. to flash the stock nd8 .tar without wiping
Are these instructions and download okay to use?:http://forum.xda-developers.com/showthread.php?t=2755793. Then just take the ota update. Then use odin to install twrp recovery
I went to Twerpme.com and found this file to be the latest one listed for my device twrp-2.8.7.0-d2spr.img.tar 8.5M. Twrp will ask if you want root, just swipe yes. Then open SuperSU
I alreadty have Super SU on the phone. Will I have to download it again?and update binaries.