I have a 5,000mah 6s 25c battery, and I usually get 8 miles off my single 50mm motor. I’d recommend you put two 5000mah in parallel to give you some extra range. The good thing about this is that you can always buy one 5,000mah 6s at first, and if you aren’t satisfied with the range it’s easy to add a second.
For the speeds you want though, you may want to go up in voltage (6s -> 8s, 10s, or 12s) to be able to get the speed you want without loosing a ton of torque. Don’t get me wrong, it’s perfectly doable with a simple 6s setup, but if you can afford higher voltages they are always worth it. Both of my first two builds were 6s, so don’t feel pressured into thinking you need all that power- especially with flats. It’s just there if you want to go that route.
An enertion kit + 245kv turnigy sk3 63mm motor and 6s should yield around the speeds you are looking for. I have a 245kv setup right now geared 13/36, and I hit 20-22mph (I believe the stock enertion gearing is 14/36 or 15/36 which will be slightly faster, giving you your desired speeds)
For a higher voltage setup (if this is something you’d want, again, it’s all personal preference/budget)
I’d get an Enertion 190kv 63mm motor (best motor out there) + 10s (two 5s in series, or an Enertion SPACE cell battery: I have the space cell, and it’s pretty legit) + either a VESC esc (it is a little more complex than plug and play, but nothing unreasonable. anyone could figure it out by reading a thread or watching a setup video) or a Torqueboard’s 145amp esc.
Again, the higher voltage means higher cost, but it’s also higher performance. If that’s not what you want to get into (I totally understand) than 6s is still pretty freakin sweet.