In order to check validity of a transaction the first thing im going to do is iterate over the entire blockchain and check if "from" address has enough "money" to transfer by calculating the sum of all tx's in which "from" address was a "to" address and comparing it to the amount that is being transferred in current transaction. That way I'll know if "from" has enough money to transfer. What else do I need to check?

Also - My transaction also holds a timestamp. I'm not sure how to use it to check a replay attack

Would appreciate any help