Honestly, from experience...know what your getting into. I didn't give it enough time myself and walked into a pre-made family looking through rose-colored glasses. It was quite a culture shock and some of the most stressful first years of our marriage. I wasn't prepared at all.
If you have to deal with your spouses ex, I would say make sure you know all you can about them and evaluate if you will be able to be apart of their life for the rest of yours. When you marry blended families, you 'marry' the ex too (at least until the children are independents).
In addition, make sure you already have a good relationship with your spouses children.
I think it is very important to talk with your future spouse about how you two will raise children in a blended family. Who disciplines, what's acceptable, etc. If you two do not stay united, the ex or children may take great advantage of that as we've had happen in our early years. Make sure your spouse is sensitive your part of their childrens lives as well. Every decision they make for their children will affect you and yours as well.
There are so many things to consider thinking from my own experience and mistakes, but it's late and I'm probably not using my brain to it's fullest potential. If I think of anything else, I will surely let you know as I wish I would have sought out advice prior. Though things are much better than they were, we had a VERY rough beginning of our marriage due to a lack of planning and communicating all of these subjects.