The problem is that there is way too much here that we don't know. I can see both sides of this problem, and to be honest it is a problem if things go wrong.
When one becomes a step-parent there is an automatic legal responsibility that comes with the deal.
It is a personal choice to take on a "used" kid. Based on my situation I can argue for both sides because my life-partner and I have been stuck financially while a true deadbeat Dad has more or less laughed in our faces. The deadbeat Dad in this case is such a total slime-ball that we are in fact the only real chance and parents that this kid had. He has asked me to legally adopt him because he wants to drop his Father's last name and adopt mine. I am the one he introduces now as Dad because he has realized that it was not his birth Father but I who in fact have been the one there for him. When he came out of anesthetic after cancer surgery I was the one in the recovery room with him.
I have taken very damaged goods from a bad situation and we have been working together to assure that he has a good life. He is now in his 20's and so many things have happened to and for him now that I have to say that in this particular case I have no regrets that I did the right thing. My life-partner and I have paid dearly in financial terms for this, but, I now know that this kid has a damned chance to make something of himself. The biological is an alcoholic that feels booze to be the most important thing in his life. The booze to him is far more important than the son he has already lost or the two daughters that hate his guts.
The word describing the anus does not really apply here. The main thing is that you have to know and understand 100 where you fit in. The problem really is not you and it really is not the child but the single Mother herself.
I myself came from a home where a divorce had split my biological parents. My own Father was a severe alcoholic and when the one I now call mine came into the picture I saw the same hurt and damage I had suffered being thrown at him on a daily basis. I did not really in the beginning want the parent thing at all, but I could not let the kid suffer horribly for a drunken sot as a Father and because he had a spineless worm for a Mother, it wasn't his fault.
In the end, I profited greatly from having the Stepparents. While I did not choose to continue any kind of relationship with my Stepmother because of some actions she took when I was in College, I remained close to my Stepfather. He had a son by a previous marriage. My Stepfather took interest in me and he took on the role of a second Dad and tried to help me where he could. He did the right thing for me and it is my duty to somehow give back to another what he did for me. When my Stepfather passed away in 1984 because I had been there, had done the right thing, and cared for him in the final two years of his life with terminal cancer and his own had not, his biological son inherited next to nothing. I as a Stepson treated him right and was surprised to find that on his passing I inherited the entire estate including investments and real estate.
He did the right thing for me, and in turn I did the right thing for him as well. In the end it went both ways and I will always remember our time together fondly and his efforts on my behalf. He was an incredibly great man and it was a privilege to have known him.
The main thing is that he never treated me as less than he should have, and in turn I gave him the same respect. I passed that on to my "used kid" and over time the same has happened there.
If I had a natural born child, there is no guarantee that the one that came from my sperm would be as good as the one that came into my life through other circumstances. As strange as it seems, I would now love them both equally.
I don't blame you for your attitude at all. You are being asked to shoulder an incredible responsibility that you did not physically create, however, if you go into it with an open mind and the attitude of all involved and with the word "all" I mean the biological Mother being the determining factor, it may change your opinion somewhat. Also remember that the lessons learned with the older stepchild will profit you greatly in trying to be a good parent to the biological child conceived with the Mother.
The main thing you have to have is agreements regarding the child long before you agree to the marriage. You would have to discuss things involving discipline and rules regarding your relationship with the child.
You must go in as a fully participating parent and not as a "meal ticket" and on that point I not only agree with your hesitancy I support it completely.
These are the issues that are really important.
If the child is given any form of control or ability to disrespect you, treating you in a role less important than that of the biologic, then you are screwed and it would destroy your relationship with your then wife.
I think that going in and setting down your feelings before the fact to the woman in question would be a good beginning point. In addition, if the situation were to be that you had no parental rights toward the child in question, or say in the raising of that child, then you would have a legitimate reason to walk away in any eyes of anyone who has ever been a parent. That places you completely in the right.
Good Luck!