Security is not bad in most towns of Angola. The worst security scenario is unfortunately in the capital itself. So be careful if you are alone. However since you intend to work for an Oil and gas Company, most likely you would be having a community living.
Housing is expensive in every town of Angola. Cost of living is very high. Angola is the costliest country in the world. While I was there I was provided furnished independent accommodation by my company and also food was free from the company, hence I was not affected. The only thing relatively cheap in Angola is LIQUOR.
Cost of all things is high, since there is no manufacturing facilities in Angola as yet. almost everything is imported including fruits and vegetables.