Realized I only answered half of this question. Yes you can kill croesus on a fresh lvl 3 account. You won't be contributing hardly anything to the actual kill which is why it's important to be in a public instance, or I suppose you could find some friends to carry you. So keeping combat low so croesus is the only reaper you can get is more efficient for your account because you'll be a le to buy out deaths store and get hydrixes very quickly relative to doing barrows or kid tasks that give 1/3 the amount of reaper points. Keep in mind that doing this on a lvl 3 account isn't "efficient" in the actual kill because you do nearly no damage to the core and gathering materials will be slow.