so what is a good level to start this DLC? can we go back and forth between NEW Vegas and the DLC? as i understand it, if i finished the main quest, i will not be able to do the DLC?

I went in at lvl 8 and lvl 30 and it worked out fine for both playthroughs so I'd say go in whenever you want to.
You cannot go back to the main game after entering the DLC, you have to finish it.
When you finish it you cannot go back to it.
If you load a save that is during the final quest (In Hoover Dam) then you cannot fast-travel out of the area and play the DLC.
If you do not have a save outside of Hoover Dam then you have to create a new character to enjoy it.

ok thanks