New point - added weight.
No matter what you do, adding a motor will add the weight of the motor, wiring, batteries, and controller. In a modern lithium-something battery this will probably be below 10 kilograms, but that's very roughly doubling the standing weight of the bike. It will feel different with that change, more-so if you mount the batteries high up.
I suggest you buy the bike you want, don't look to mod a brand new bike. A new ebike would have the batteries placed inside the downtube, or aft of the seat tube. This helps overall weight distribution.
A retrofit kit might use up a bottle cage mount which isn't good because as noted, batteries are heavy and two little M5 bolts are barely up to the task.
Other retrofit kits include a carrier/rack and locate the batteries high over the rear axle. This is particularly sub-optimal because the added weight is 100% on the rear, which is moving the balance away from the 60/40 ideal.
TL:DR; get the ebike you want - buying a bike to retrofit an electrification kit will result in a poorer bike than one built to be electrified.
Rephrasing that on the question - look for a bike that lets you mount the batteries inside the frame, or in a properly secured way that is low on the frame. Not a bottle cage mount, and not a rack mount. Downside - they don't exist, so if you want an ebike, buy an ebike.