I usually strongly advise against fanless PSUs in servers, since the extra heat will make keeping the drives cool more difficult.
4GB is plenty, go with 8GB at most. Performance is mostly irrelevant. As long as you avoid the ultra-tiny (physically speaking) and the ultra-cheap drives, you'll be fine in all cases. Remember FreeNAS runs from RAM - the drive is not accessed regularly during use.
I'd recommend against Kingston, due to their shady practices as of late. Check ASRock's Memory list for specific RAM that's been tested.