Should I have to pay her for the lifetime if she can’t marry with anyone?
No need to pay maintenance to her for life time, it is only for “Iddat Period (3 months)” in addition to "Meher" and some amount as per Quran, if you pronounce Talaq to her;
Is divorce necessary for re-marriage in muslim personal law?
Divorce is not necessary if you want to second marriage; but it is necessary in case of re-marriage for your wife.
After marriage, if 1 or 2 children I’m having and my income is not such then still I have to give maintenance?
If you are marrying second, then you need to pay maintenance only to “Son” of your first wife, if you pronounce talaq in addition to reply of first query.
We are muslim, Is any another law for me regarding said case.
Need clarification for this query; As per the circumstance of cases Indian Law will be applicable to you.
In Jharkhand state, what will be maximum amount per month paid under Section 125 of Code of Criminal Procedure 1973.
It depends upon the circumstances, your income and requirement of the petitioners (wife, till you pronounce Talaq and for son)