Been there - done it. It was a very nerve racking experience. Every immigration official we spoke with gave us conflicting advice.
If you go for the 2 year spouse visa and wish to apply for a perm. visa after the 2 years are up. On the application form, when they ask for documentary evidence, they are not so concerned with quantity as much as they want to see a document for ever single month for both of you. We did not provide documents for one month but had numerous for the months either side (by mistake not design).
As a result we had to sit an interview with an immigration official who made Hitler look open-minded. Got it in the end but they really don't want immigrants here and they make that quite clear.