Reaz,
Housing is expensive in Dar and getting more so by the day, there is a real demand for rented properties especially by Expats working for big organisations who are being asked and can afford to pay over the odds. Asking for a full years rent in advance is now common place in the family housing market. In my view there is only one school worth considering...the International School whose Headmistress is a very competant American. For very good reasons pay and conditions at Coastal best suite single people so I would suggest your wife would need to look at working fulltime....but getting her a Work Permit would be a story. Cost of living for a family in decent housing with kids at the Int School works out at about US$4k pm then you need to think about a vehicle fuel food etc. Water for most Dar residences needs to be brought in by water tanker and although the electricity supply has much improved you would need to make sure any rpoperty rented has a functioning generator.
Other than that life in Dar can be really good and flying with Coastal interesting. By my standards they are a good outfit, with well maintained Caravans and compared to other operators some pretty sound revenue streams.
Go for it!