Newb here! Cheaper to build wheel or buy a kit!?

I don't know of a readily available hub motor where you get just the hub part and not already mounted in a wheel. Well there may be, I recall some time ago at crystalyte's home page being able to order just the hub motor not already mounted.

In any case .. not knowing how to build wheels on my own I think it's easier to just buy the kit. FWIW I read up how to do that in Zinn's book on mountain bike maintenance. It looks like one of those seemingly simple things that may be daunting in practice. I'm sure there's some possible advantage to building the wheel yourself such as being able to use the best spokes or whatnot.

A vendor on ebay sells just the motor, for bmc gearmotors. Several crystalyte vendors sell just the motor too.
Ebikes-ca is the best vendor for chrystalyte, but only the 404 is sold without a rim. They may allow you to buy other motors if you call them. Electric rider sells the motor without a rim. Another gearmotor seller is Holmes Hobbies where you can get bafang hubs.

A real good wheel can be built yourself, or custom built for you, that includes much better spokes and rims than ordinarlily found on kits. So if that matters to you, you won't be real happy when you see some of the ebike kit wheels. One of the cheapest kits, Wilderness Energy will still allow you plenty of time on the truing stand. Typical chinese kits come with a really cheap but strong rim, and 12 guage spokes. They can rarely be run without some serious straightening.

As for cheaper, not if you have to buy a bunch of expensive stuff to complete the kit. If price is the issue here, the best current deal on a serious ebike is an aotema front hub kit from High Tech Bikes, with a lifepo4 battery. Less than $900 will get you a bike that will go 23 mph for 23 miles on a charge. I'm currently trying to destruct test one of those hubs, and so far, it's performing well. Rear hub is generally prefered for a lot of good reasons, but front hub is cheapest. The absolute cheapest, is ebay Wilderness Energy kits with sla batteries, under $400. The WE kits use the same Aotema motor, but come with sla batteries that can go about 6 miles.

Lots of folks find buying the kit a pretty good way to get all the parts, and have the plugs match up and such, and then just relace a good rim on the hub. Buying just the motor would work best if you are replacing a motor, and have everything else already.