I think that if you are being responsible, considering the financial risk of (the possibility of) having twins is very, very smart.
My guess is that you would not choose selective termination. It's good to know where you stand going into this.
I posted on your question yesterday and still feel that we all have our own path to follow. I had three previous miscarriages before having my son; I can also say that if we had been determined to have another child, that wouldn't have stopped me. Yes, it was *a* factor in our deciding not to have a second child, (knowing that I wasn't going to have to go through that heartbreak another time around or more... I know women who have had ten miscarriages and still had healthy little ones later), but our financial situation was the bigger factor. I don't believe in 'Have babies and let the universe provide'... I find that philosophy highly irresponsible as I grew up in poverty with little to no money for extracurriculars, lessons, preventative medical care, dental care, etc. (I was taken to the doctors maybe three times between grade 4 and high school). And, we really felt one kid was right for us. But I do understand the desire for more. It's not wrong to want more, it's just that it is a very different path of pregnancy. Expect to have your pregnancy very highly managed by doctors. I wouldn't have been cool with that, but to each their own.
So, if you are feeling that this process is going to cause more anxiety and possibly financial hardship, once again... really look into your options. Talk to a fertility counselor or a regular counselor. I think this is a very legitimate area of life to feel conflicted in. And as others have said, please get married first. This is a very expensive process to undertake and you want to have as much financial protection as possible. Getting married will really help in that area. (I say this as a woman who became a mother before legally marrying my husband-- I don't have judgment around this, but do know that it's much easier come tax time, etc.)
and Wild Woman gave you some good advice. This isn't a decision to rush into... get more information. Doctors are sometimes wrong! Endocrinologist is your next stop. :)