I parked myself in a corner of the nearby Starbucks to change the drive. Starbucks has electric outlets galore, where I could plug the adapters for both the MacBook and the external drive that booted Mac OS while the hard drive was out of commission. Kudos to Starbucks also for leaving me alone; no one asked what I was doing with a mess of cables, screwdrivers, boxes, and parts strewn across a table meant for lattes and frappucinos.
Unfortunately, the new hard drive wasn't recognized by the computer. Next I'll try replacing the SATA cable (bottom left in the photo), which various tech sites suggest could be the problem. Also unfortunately, Fry's didn't have the cable, which had to be ordered from Amazon. Resolution (repair or replace the laptop) would have to wait a couple of days.
[Update - 5/10: the new cable worked. The total cost of the fix was $120, much less than a new computer. The latte and frappucino budget has been adjusted upward accordingly.]