The worst part was filing down the metal brake assembly so it didn't rub the hub motor on the rear wheel. I assume that was a very unique compatibility issue. Besides that, it was no more difficult than assembling a computer, just a bit of screwing and wrenching which any bike owner would be able to do (add in some spitting and cursing if you've never worked on a bike before, as I hadn't). Done in a weekend.
The second hardest part was figuring out what to buy. The most straightforward option is a full hub motor conversion kit, which should work for any commuter application. Trying to individually buy parts (likely from aliexpress) is cheaper but a hassle to coordinate, since there's no standard for ebike part compatibility. Then there's also the choice between PAS and torque-sensing, throttle, etc.
I ended up buying the minimal throttle conversion kit from Grintech (~$1200 total) and installing it on a used hardtail mountain bike I got for $200. The bike goes ~26mph on a 48V battery, 30mi range, works like a charm.