First of all, let me congratulate you on your offer. It's always great to receive new colleagues at McKinsey, especially when it's so close from us (Luanda and Lisbon shared the same roots until very very recently; even today, most of the Luanda's leadership was transferred from Lisbon).
Let me now address both of your questions, which I believe I can do quite accurately as I spent one year working there!
1. Honestly, I would stick with the same advices I give to everyone who is in your position, regardless of the office: enjoy life as much as you can before starting your work! Don't bother developing any skills at this stage, you'll have plenty of time during your McKinsey journey, believe me :)
2. In Angola, most of your work is going to be done within the country (90%+), more especifically in Luanda (very rare exceptions of projects outside the capital; depending on the cycle of the economy you might need to do some more projects abroad).
Lifestyle is (much) better than Southern European offices, you should expect a 11/12h daily journey tops (with some exceptions).
For me, one of the biggest differences was related with the skillset that you will need to master: it's important that you're somehow good at basic technical skills (math, excel, ppt), but they definitely require more from you on a soft skill perspective (a lot of client handling!!). Try to start establishing good relationships with clients since day 1 :)
Hope this helps... and most likely will see you sometime in the future as McKinsey has a client there in an industry that I like, so will try to do a project again there!