How much is essential, depends on the weather and time of the year, but a drysuit can be worn year 'round. You just may have to do more practice rolls than usual if it gets hot.
For what it's worth, every time I hit up the Poudre, it's been in my Stohlquist Bpod.
Always wear socks if the drysuit has those built-in sock/booties (or your feet will get gooey). I'll wear an IR union suit underneath if it's cold, or just a thin nylon/polyester shirt and leggings if it's normal summer kayaking temps.
Bombergear creek mitts (pogies), because they have a wide-opening and slip on easily, and still offer pleny of protection. You'll appreciate pogies with how cold the water gets.
Any drysuit brand is fine in my book, because I'm willing to buy a bottle of Grangers XT and maintain them. Kokatat lasts the longest off the rack without needing maintenance, though.
I found my Stohlquist on clearance and it's a size too big, but it's been working out great.
Another trick for saving some money on a dry suit, buy a scuba dry suit. You have a lot more affordable options and you can always cut and patch any scuba-specific parts you don't want.