Welcome to the Nexus club
Here are the answers to your question;
Rooting erases all the previous data and apps? NO. The process of rooting only provides you with root access. (But dont be so happy yet
As I see, you have a stock Nexus. So before Rooting, you have to do a process called Unlocking. And Unlocking wipes EVERYTHING.
Once rooted and unlocked, when you flash switch ROM's you have to do a clean flash. This removes Apps. But preserves app data and sdcard contents.
For backup of data, I'd recommend go for individual apps. The below are my personal favorites for backups:
For app backup, use ES File explorer (It only backs up free app, and no data backup)
For SMS and log backup, use SMS Backup & Restore and Call Logs Backup & Restore by Ritesh Sahu
Contacts and calendar and mails will anyways be synced with google account.
Do all the backups and copy these contents to your PC and then proceed with the Unlocking and Rooting.
Once Rooted, you can use Titanium Backup or ROM Toolbox for data and app backup.
i have 4.1.1 currently on device. Does it mean after root only 4.1.1 roms will work on my handset. NO
When I bought my GNex, i was on 4.1.1 too. Currently I am on 4,2.2 ROM's
Hope this helps.
All the best.