miniaturization costs more. the screen is more expensive since it's higher DPI at the same resolution. more engineering has to go into a smaller chassis to keep it cool and get all the same stuff in there.
I'm not sure there are XPS15s for $700. There's an XPS13 for $800, and those are really good notebooks. I checked newegg and there's an acer and a Lenovo (i like lenovo laptops also) that have 8GB of RAM, 256GB ssd, 15 inch screen etc.
If you want to go with the Envy, I'd just recommend getting the 256GB SSD upgrade for $60. 6GB of RAM will serve you fine- it's easy to upgrade later if you find you need to, and cheaper to upgrade later too.
if it has an M.2 slot, you might be able to just add it and have both a SSD and a HDD. the HDD is not in M.2 form factor. it's 2.5inch, 7mm height most likely. you could replace it with a 2.5inch, 7mm height SSD if you wanted to.