Hi M.,
It seems to me that your BF's timeline is not that outrageous and your math is a little off. Here's the way I see it.
You're having a long distance relationship now -- have you spent much actual time together? If not, dating for 1 - 2 years while living separately makes sense. This give you both time to really get to know each other in person, not just over the phone, and it gives your kids an opportunity to get to know him, too. If it turns out you're not as compatible as you thought (that's the dual you, by the way), or if significant issues arise with your kids and him, then you're not in a situation where one of you has to move out, which would be far less traumatic in the long run, especially for the kids.
Ok, moving here and waiting a year or two to marry puts you at 35 - 36 years old. That's not unreasonable. Now, let's say you wait a couple of years before having another child. This give you both the time to know that you're compatible living together as a couple and that there won't be strife with the kids (ha, like that's a realistic expectation with teens! ;-) ). That seems very reasonable to me. My hubby and I were married for 6 years before we decided the time was right to have children. Yes, we were 28 at that time, but the time together cemented our relationship before bringing a little bundle of stress, um, joy, into our lives.
So, two more years until pregnancy puts you at 37 - 38 years old, your daughter at 19-20 and your son at 12-13. There are slightly greater risks of having a baby with a genetic disease at this age, and your OB would probably want you to undergo amniocentesis, but other than that, lots of women have babies in their late 30's and early, even mid, 40's. Your daughter, in the meantime, will be pretty much grown and doing her own thing (maybe even out of the house and away at college by then) and your son will be at the stage where he'll be trying to identify more with his friends than you, in which case it will almost be like raising an only child. I had a friend who deliberately waited until her son was 12 before having a second child. There was absolute peace in the family -- no sibling rivalry b/n the kids and her son absolutely doted on his baby sister. (He was a great live-in baby sitter for several years, too!)
So, the question comes down to: do you love him? Do you think he loves you? Are you both committed to working at making a relationship work? If so, considering that he's not moving in from the get go which leaves you both options if things don't work out as planned, then why not give it a try?
Good luck to you -- I hope everything works out the way you want it to.
R.