Cleric 6
Has a BAB of +4
Concentration is a class skill; 8 points by lv 5
At this level, earliest possible, can take craft magic arms and armor
Has turn undead by level 1.
Is what a paladin wants to be.
Unless you can get the turn undead and a bonus feat at lv 5, this is the earliest feat-wise you can do it. So you'd enter by 7.
What, pray tell, is this PrC you are aiming towards? Because at cleric 6, there's a lot of good you can do. Hell, there's a lot of good if you wanna go cleric 20 or PrC out starting at ecl 6.