I don't have any knowledge of LP fireplaces, but have found that the secrete for a good fireplace is in how heavy it is. All the ones that people complain about to me are light weight. I personally have a wood stove and absolutely love it. I even enjoy cleaning the ash out of it because it's part of the process that is such a joy living out in the country. From cutting up the logs, splitting and stacking the wood, to enjoying the fire in winter, I love every part of it!!!
If I was buying a LP fireplace, I would start with the local suppliers and ask them about their better models. Not the most popular, or the cheapest. For a fireplace, it's worthwhile spending whatever it costs for quality. Then I would google the brand and model they recommended to see what I could find out about them online. Read some reviews and find some complaints. Amazon always has the best reviews, but there are other sites out there like this one that will specialize in fireplaces and wood working/living off the land, with reviews.
Your builder would be a good source to find the supplier, but probably not so much for what model to chose. Take his advice, along with everyone else, and do your research.