STEP-MARRIAGE

hello all. i am in love with someone i've known for over 25 years. our relationship is 'rather' long distance but we are very much connected emotionally. the problem i have is that now that we are seriously talking about marriage and after getting all caught up in the whole 'getting engaged and married' thing, i am now having second and third thoughts about it.
we are both in our late 40's and have been married before and 2 children each. i should start off by saying that i had no intention of starting a new relationship with him or anyone else when we began corresponding but we did have feelings for eachother many years ago so i guess those old flames started again but i have been single and celibate for over 6 yrs and have been comfortable without any desire to change that status. anyway... i have one teen daughter and a daughter in college. he has 2 teen sons.
i have been trying to relocate for a year now and he would be relocating to be with me too. the problem is that while i have raised both my girls as a single mom, i have ABSOLUTELY NO DESIRE to raise anymore children. i feel it would be unfair to continue in a relationship with him knowing full well that i do not want to be a step-mom.
i have tried making my concerns known to him but he always dismisses them as jitters and tells me how nice and well behaved his boys are. his youngest wants to live with him when he relocates because he has been in a different state than him all his life.
i have other issues about this too like... my intention has always been to relocate with my youngest daughter and rent a 3 bdrm home for me and my girls. when the relationship progressed i included him in those plans but i stopped short of realizing he also has a ready made family. he says he'd like to buy a house within a yr or two... i had not considered that before he mentioned it. what happens if something happens to one of us? my kids vs. his kids? it is all too complicated and i am at a point where i want out. all my kids have is me where his kids have both parents. i feel like the short stick somehow... too many variables to consider - social security, etc.. HELP! can anyone work with me thru this?