Well, both VESC and Hobbywing systems will let you turn it down. More power also means more braking. A bigger battery is able provide a bigger resistive load, which is required for stopping power. The regen braking on my Eovan is strong enough to lose traction on some surfaces.

So you'll prefer overspeccing a little.

I've not looked at kits, except the revel kit. Which seems to have disappeared. If you can find one, they're know to be great.

For a full DIY conversion I'd be finding and vetting each component individually.