is this for 2.3 or 1.9? i dont know what the differences are in npc scripts between them but on 1.9 @CHECKRENEWLEVEL 1 would check if the player is Rebirth x1 @CHECKRENEWLEVEL 2 would check if the player is Rebirth x2 and so on
for the quests you could either use flags ( @SET 300 [1] would set a flag and @CHECK 300 [1], i think thats right but a quick search on flags would benefit you more than just this) or you could use the @ADDNAMELIST and @CHECKNAMELIST commands, again a quick earch will give you more info
FLAG
[Main]
#IF
CHECK 300 [0]
#ACT
goto @quest
#ELSEACT
goto questdone
[@quest]
quest script goes here
#ACT
SET 300 [1]
[@questdone]
#SAY
You have already completed this quest
NAMELIST
[Main]
#IF
CHECKNAMELIST quest.txt
#ACT
goto @questdone
#ELSEACT
goto quest
[@quest]
#SAY
Would you like to start this quest? blah blah
#ACT
ADDNAMELIST quest.txt
[@questdone]
#SAY
You have already completed this quest
I dont know if this information 100% correct i aint done anythin with mir at all for like a month but i know there are threads on both commands as i learned from them myself. As for the mobkill part, i never really looked into that much myself yet