New Build vs Existing Home

I am frustrated.  We have been getting quotes from builders for the house we want - and the range is huge.  So now I'm busy comparing those, and what would have to be upgraded, and what I could live with, and what must be done at build because it is impossible or prohibitively expensive to retrofit later.  And while I'm doing all this, I am looking at existing properties... and thinking about the tradeoffs involved between having 6+ acres, and having 10-18 acres... and between the house we want, and the house we can afford to build, and the house that is 45 years old on the larger property that already has a barn, fenced pasture, a shed, etc., all of which has been very well-cared for and is in great shape...but which has only 1.5 baths, and no walk-in closets, and is smaller than our last house, with a basement that cannot be finished... but it has a big, beautiful, hickory kitchen, and a lovely three-seasons room with a wood stove, and real hardwood floors, and is hunt-able (there are 5 deerstands on it already), and it backs onto state gamelands directly... where the property we own, we have to cross the road, or cross the neighbor's lot for access...our property has no woodlot, the existing property has 5-6 acres of trees... Do you see why this is hard? 

Do we want a large percentage of our monthly income going out in mortgage payments?  (Not really).  Would we rather have it be 15% of our net?  So we have $$$ left for other things?  (I would).  Do I want to share a regular closet with El Jefe?  (Not on your life).  And there are the other issues.  Our property is fairly close to everything we do right now.  The existing property is not.  Our property has neighbors with lots of kids the age of our own.  The existing property - I don't think so.  But we could be a whole lot more independent a whole lot faster on the existing property than we could on our own.  And the kids could have a horse... if I completely lost my mind.  Or I could raise a feeder pig every year and a steer every other year, just in case my parents come to visit and Mom doesn't want venison and Dad doesn't want rabbit and it isn't wild turkey season...