FYI you’ll have one response with a low purchase price and high repair cost. This was technically a voluntary battery upgrade.

chevy spark, Geo Metro,

It depends on the type of conversion you’re doing.

A FWD vehicle is an excellent candidate for Prius or Leaf drivetrains. Batteries under the back seat and truck.

A manual transmission vehicle can use just about any electric motor as the input if it meets the power requirements.

A vehicle with independent rear suspension can easily (weld in new mounts and axles) accept an drive unit from most production EVs.

Having high ground clearance, long hood (Porsche, 240/280z), or long rear (wagon) allows for larger battery boxes.

So basically if it’s not a really short unibody vehicle it’s not too bad.