I notice your budget is creeping upwards. That's a good thing.
You need to be checking shipping costs when checking out kits, most ship from China and you will be impressed at the cost. You are not getting accurate pricing info unless you are adding shipping. The linked kit will go well over your budget amount delivered.
Most performance specs are quoted with a 26" wheel. Paul's info on that page is good stuff. Read it, everything he has. Think about it, then go back and read it again. He is a highly regarded vendor here, very few reports of problems, none that I have seen having gone un-resolved, and I read a LOT. He sells a lots of kits, many here who have purchased one. Plus, English is his primary language, SFAIK.
DD drives can be somewhat more reliable, fewer moving parts than geared. Performance enhancements common. They are also much heavier than geared, generally, and induce a parasitic drag when pedaled un-powered than can be significant.
You are most likely not going to be pedal-assisting any motor much past roughly 10% of it's rated top speed. Most motors are providing no boost at all past this point. If you can't pedal your current bike at 30mph, a 20mph top-speed motor isn't going to help any, and it will affect your options on changing gearing, at least a rear hub will.
Not very many people here electrifying road bikes, most use something closer to a mountain bike. Many here pedal little or rarely. Users Chas58 and Russel(something) have done something similar to your needs, I recommend you check out their builds and performance and usage results.
The MAC may be right up your alley, perhaps a Cute. I ended up with something in-between. Wanted light weight and have no hills to climb. Willing to extend range with pedaling, high speed not needed, also can assist for short hills, I went with a 4-5 lb hub and a 36V15a battery. 30 miles at 15, 18.5 top speed, 20+ miles at top speed. Been very happy.
