The regiment he joins will indeed affect how much time he is away. He should be sponsored by a particular cap badge if he has a place at RMAS but that is no guarantee that he will join that regiment or corps (my husband was sponsored by one cap badge and ended up in a completely different one). It is probably fair to say that there all cap badges are stretched at the moment and there are a lot of operational tours to be endured.
Technically, you will move on posting every two years but that can change with promotion courses/short tours etc. We had six postings and two op tours in our first five years of married life.
It can be difficult to plan into the future - we organised our wedding around one set of circumstances and found ourselves dealing with a completely different scenario so good insurance is vital!
We've never managed to be posted into the same area for longer than two years and given my husband's corps, we're unlikely ever to do so - doesn't meant that it isn't possible, though. I don't know anyone who has managed it so far. Notice for a move has varied from two weeks to eight months and a variety in between.
The Army will house you if you want to move but only if you're married. There is no provision made for couple who cohabit.
We've been in quarters (some nice, some hideous) and we currently own our own home. We wanted some stability while our children were small and we choose to stay near my family and my husband commutes as and when he can. So, it is possible to own your own house but there are advantages to living on/near camp as well.
I can't offer any advice regarding a career as I didn't have one (I didn't have a very portable job before we married) and I simply picked up work (paid and voluntary) wherever we were posted if I felt like it. There are rules regarding running a business from your quarter but I'm sure there are people who can give you more idea of the red tape involved.
I can't really give you the pros and cons of being an Army wife - I can only give you the pros and cons of being married to my husband and that might not hold true for your relationship. I've never really struggled with us being apart as I'm pretty independent and find it quite stifling when my husband is around all the time. There is really no "one size fits all" list when it comes to marrying someone in the Forces and what I consider a "pro" might very well be a "con" for you.