I recently, on one of the 12 days of Christmas on MicrosoftStore.com purchased a plastic, not metal (Envy) Pavalion 13-p111nr x2
for $599 and it now vacilates between list of $899 and $499 and going out of stock quickly at $499 once in the last two or three weeks I had it in my cart and they went Out Of Stock and refunded my VISA while trying to process.
I successfully added the 2nd HDD in the keyboard base by buy HDD Hardware Kit and a new HDMI board (one shipped with it had a missing surface mounted connector and one chip missing also) and a 500GB 5mm single plater Seagate HDD from Overstock.com - for about $190 total using hp.com/buy/parts and asking for free shipping. It was an adventure. Only wish it had a backlight on the KB and a bit brighter and FHD disply.
As you mentioned, the trick with these is that there are so many different subtle variations of these "x2" models, with minor differences between each, so it's hard to determine exactly what your upgrade options might be.
I did the same as you, except with the Black Friday version from Best Buy (metal, i3-4010Y, and looks similar to this woot, model was 13-m110dx e8c05ua#aba).
In the screen portion, that one had a single DIMM slot so I removed the 4GB and put in 8GB which fit, though rather tightly (it seemed to want a just minutely thinner DIMM module), plus an mSATA slot, so I pulled the 128G and put in a 240GB, and a microSD slot, which can come in handy.
The base could accept a 7mm drive just fine, but I also needed the drive installation hardware kit (includes cable and "caddy") and a replacement HDMI board that has the chip and connector for a hard drive (from the HP parts store; found a coupon code at the time).
Basically, with regard to upgrades, YMMV.
Also, none of those parts should really be considered end-user upgradeable, since they require significant disassembly of the unit and it would be pretty easy to damage it by doing so if you're not careful. Certainly not as bad as trying to take apart an Apple product, but also not nearly as easily serviceable as a traditional laptop either.
For what it is, it works well, but it seems like kind of a niche product, primarily for people who want to have-their-tablet-and-their-laptop-too, without breaking the bank, and who can handle a little more bulk than a smaller tablet.