My first blush thought about reading your post was you're talking to an agent for the builder without the benefit of having an agent working for you. I assumed you were doing this to "save money". While it's certainly possible you can DIY with the agent for the seller, you should remember who he/she works for. They owe the builder a fiduciary obligation and honesty and fair dealing to you.
I thought it was too late to offer my advice when I read it, and it may well be if you've already made the appointment. You may want to hire a Realtor for an hourly fee to advise you on this.
As a buyer you do have some leverage in today's real estate market. bargain hard and ask for the moon, you just may get it.
Good Luck
