No idea about taxes since these are usually taken care of by companies. Ask the company to quote salary after deduction of taxes. Never heard about the spending 40% rule.
No idea about housing too ... generally safe here but you have to take precautions like anywhere else .. & also depends on the locality.
Tripoli is quite safe for females if you take necessary precautions ... since your daughters are of school-going age, they should be escorted (public transport not good) and should not take taxis by themselves. Most schools have buses they could use or you would have to hire some regular private transportation/ a driver for them (through a known person).