I think the best and the cheapest available option would be a standard exterior door having gaskets around the edges, the seal at the threshold. It is 10x heavier than a standard interior hollow core door...you will notice a difference. Check them online, you will get various deals associated too.

I have masonite bellagio doors in my house. I like them. Only thing that was a problem was lead time, and it was due to what I ordered. I ordered extra thick doors for the sound deadening properties. It took forever and kept getting pushed back. Also this was through Home Depot. End result is good, and on these the panel projects from the slab so nice look, however we had a curten as our bathroom door for a month.

they do tend to swell some but I dont know if its worse than wood. Here all that is used in the interior is the molded doors most of the time. however no matter what door you use with the proper installation the doors will work fine. Some minor adjusting might be needed after the doors have been set but not anything major

I think your GC is a little off. We just did a high end house with Integrity/Wood. Unless they have changed recently, the Ultrex/Wood looks a lot nicer on the outside also. See the link below to our house. The Wood/Ultrex turned out really well. These pics are at the end of construction, they look even better now. I have had these for a little over a year now and am very pleased with them. Our builder leaned towards "cheaper" windows also (he preferred WIndsor). To me, Windows are one of the most important aspects of construction. You are talking $5k or so on an $800k house. Don't scrimp on windows, doors or plumbing fixtures. Here is a link that might be useful: Link to our house with Integrity Ultrex/Wood This post was edited by lkbum on Thu, Apr 4, 13 at 14:22