Front hub kits are nice value -- easy to install and low cost.... so if you want a hub kit, look for low cost options (with the power rating you want) for the hub kit. note - Chinese ratings reported tend to be inflated. Please realize that you won't be jumping curbs once you have a front wheel hub kit installed.
For batteries, you can save a lot by using a smaller battery, unless you know for certain that you will be riding for more than 1 hr at a time. 1 hour will get you quite far... I found that on a 1 hour battery, I would get 30 minutes with lots of hills and my own pedal power for the downhill / flats OR, just over an hour using the throttle on downhill only. The only problem with a smaller battery is that continually completely discharging it can shorten its life.
I have pretty much convinced myself to buy the bafang mid drive if DH will install it for me, but that costs a LOT! more than a hub kit.