non0, Everything is tax free. If you stay in company accomodation you won't see any bills, although if you exceed the utilities allowance the balance will be deducted from your salary (never actually checked).
Unless you have a large mortgage at home, or other payments, you will not have that many expenses in Doha apart from your car and food. That is unless you have more than one kid, in which case you will be out of pocket for school fees.
What is 401k? Can't answer that one.
F/Os now get the 10k housing allowance and are moving out of company accomodation as we speak.
Company accomodation is the luck of the draw, but at least everybody has the option of renting their own place. Never buy!