HP LaserJet is the longest-lived product line in computer printers.
They're durable, and make good "used" printers. You shouldn't get the really old ones, because of interface issues. To use the old printers, you either need to stick with a parallel port, or Ethernet. If the printer has an EIO port, buy an old JetDirect print server for $15 and install it. Older printers do not have USB!
The early printers were named LaserJet, 2, 3, 4, 5, with letters following. The oldest you should consider is the 4, but with an expectation that it'll break soon. The 5 series is tougher, and generally may last a while.
The 4000 series followed the 5, and represent the first real great series of used LaserJets. These are selling for under $50. They have great toner life, and low toner cost. You can install an EIO network card, and you're set.
The 5000 series were like the 4000, but with a larger format. The 8000 series were networked office printers. The 5000s are worth getting, if you can find them. The 8000s are too big for home use.
The 2100 series were lower-end printers, introduced in the late 90s. There aren't many around anymore. The series was incremented to 2200, 2300, etc. Now the line is called P3000.
After the 2100s, HP intro'd the 1100s, which were even cheaper. It's turning out that some of these are pretty rugged and still in the used market.
For whatever reason, the lower-end used printers are still pretty expensive compared to the 4000 series. This makes no sense, because the 4000s will outlast the newer low-end lines. The successor to the 4000s is the P4000 series, and also the newer 5000 series.
A feature to keep an eye out for is PCL6 which came out with the 4000 series and all subsequent printers. PCL 6 is likely to be supported for a long time. PCL5 may not be supported in the future. Todays computers will handle it, but a few more operating system changes, and you may discover that these old drivers are not going to be ported.