My aunt gave me some pretty good advice when I was first dating my now-husband. She said, if he gained 100 pounds and was in a wheelchair, would you still feel the same about him? If the answer is no, then he's not the one. At the time I didn't really understand what she meant, but now that I've been with my husband for 10 years, I get it. Marriage is so full of ups and downs - there will be times when one or the other of you is sick, injured, out of work etc. - so there has to be something there where the two of you love EACH OTHER and not the idea of each other. Also realize that love changes with time, and this is not a bad thing! Some of that intense spark will go away, but underneath that (if you're lucky) is something softer and better.
I think some of the basics are - does he love your kids like they're his own? Does he treat you with love and respect, as his equal? Do you agree with him on how many more kids you will have (if any) and how you will raise them? Are there any other "deal breakers" that you have? If nothing else, there is no harm in having a long engagement. Don't get so caught up in planning your wedding that you forget to plan your marriage! =)
Good luck and best wishes!