As for the cost of living, some things are more expensive, and other much cheaper. Since everyone has different standards, different expenses, and different goals, I couldn't possibly vouch for your ability to live well and still save significant money, but yes it is possible.
Partners may stay with during assessment (hotel), however as far as I know the company will not provide airfare for them. You will be fairly busy during the day, and the format of the assessment doesn't allow for them to accompany you to the HQ. More bluntly, there are a lot of pilots looking to join from all parts of the world, so the company while offering lots of information and assistance in getting you settled and started, is not in the habit of trying to "sell" Abu Dhabi/UAE to western candidates.
As far as fleets and seat locks, it's another difficult question to answer. Traditionally the fleets were divided Airbus/Boeing, and a fleet change was something that happened only occasionally and very senior.
Airbus hires either by type or necessity. In your example of hired 320 FO, eventually as your seniority and company minimums permitted you would mix fleet with 330. Then again with seniority if you desired you could keep 330 and mix fleet with 340. There are FO's who hired direct to the wide body with previous experience. Upgrades were to the left seat 320 regardless of current fleet. If the company mandates you change fleets, there isn't a seat lock per se, but if you are close to your upgrade for instance they may not grant you a fleet change without a seat lock (or you can bypass and wait to upgrade if you prefer). With the addition of the 380, and due to the massive training costs involved, the company put a 3 year seat lock on all 380 bids. This meant the super jumbo went very junior, as all the senior guys didn't want to belay their command. Add into the mix the 330 Freighter and 321, and in a few years the 350 and things complicate further.
On the Boeing side, 787 is going very junior as well as most guys want to stay 777.