The retrofit is mainly why I'm interested. I have an 04/07 build w/o cooler and made a similar decision to do it myself, not purely because of cost but also because Stett and Advan wouldn't respond to my requests to purchase their kits. Hoses and fittings are easy to get a deal on, even 634 cores come up on ebay really, really cheap once in a while.
As far as the "engineered" kit vs not "engineered" diy, that is a terrible argument to spend more or less on a product. Engineered is not a word that describes quality, something could be well engineered, or poorly engineered, or engineered to be just cheap enough that a company can make money on it. There are engineers on this forum who have done similar diy oil coolers instead of paying ER for branding and packaging. Without the 2nd core or shrouds I don't see the point of the ER. In fact I think the t-stat housing delete with bigger AN fittings like the stett and advan kits is a better "engineered" solution than ER or Turner or ECS tuning oil cooler packages.