Most bikes are fairly common and share a lot of the same components and configurations. If you do not have a bike ready, I highly recommend purchasing a brand name bicycle, such as Trek, Fuji, Cannondale, Diamondback, Surly, Santa Cruz, Norco, Canyon, Scott, Yeti, Marin, Specialized, Kona.... I could go on but most department store brands are not "brand name" unless it is older Schwinn. There are plenty of places to purchase quality name brand bikes such as Craigslist and Facebook marketplace. Your Local Bike Shop may also sell used bikes as well so check them out and give them support!
You will need to determine if you want full suspension or hard tail. Full suspension has front and rear suspension components and hardtail has a front fork no rear suspension. Full rigid bikes are just that no suspension at all. Full suspension bikes will need a little more work to convert as you have to consider you have moving parts of the frame so you will need to allow proper wire routing and grooming. Best pick bike suitable for your needs.
Another important detail on bike selection is knowing what axles you have on the bike. Now this is only important if you decide to do a hub motor build. Almost all bikes will accept a mid drive motor with a few rules as exceptions (I will cover those rules shortly). Your common bicycle and pre 2015ish mountain bikes will typically come with a 100mm front axle and 135mm quick release axles. These are very common and many hub motor manufacturers make wheels that will fit your frame's axle mounts (usually called dropouts). Most of your newer mountain bikes, some newer road bikes and a lot of your fat bikes will have what's called a thru axle. These can not be convert to hub motors (you can but its a lot more $$$) and usually will require a mid drive motor installed. Usually common thru axle sizing is 12mm. Fat bikes can vary in axle sizing, you can look up on google what your axle is usually with the model name of bike. Make sure you match the axle sizing with the new hub motor, sometimes these can be spaced out within5-10mm each side in your dropout.
For your mid drive builds you need to pay attention to the bottom bracket sizing on your bike. Most standard bikes have a 68/73 common size bottom bracket shell. This is the most common mid drive motor sizing and can be purchased virtually anywhere. Newer mountain bikes might have a bigger BB shell such as a press fit 92. Press fit means the style of BB (pressed in not screwed in) and 92 is the width of the bottom bracket. These are common on Boost axle spacing bikes, mainly mountain bikes. These frames will require a 100MM mid drive motor and this axle adaptor. Your fat bikes can either have a 100MM bottom bracket OR 120MM bottom bracket, so make sure if you are building a fat bike to get the right mid drive motor for your bike. If you happen to have a 68/73 mid drive motor and need it extended, there is a kit you can buy. If you have a PF86/92 with axle sizing 41 you will need this kit. You can avoid these extension kits by buying the correct mid drive motor axle size, remember - 68/72, 100MM, and 120MM. Empoweredcycles.com has a lot of kits and tools for your Bafang middrive conversion and has quick shipping.
This should cover you to get started, if you have any questions about bike set up or build feel free to email me or text me and I will help you as best as I can.
If you feel you are ready check out our Conversions page to see list of options