It's not a Defender. This always sounds pedantic, but the "Defender" name did not come into use until 1990. All vehicles produced prior to 1990 that otherwise appear to be what we think of as Defenders are either a Land Rover 90, Land Rover 110, or Land Rover 130. The Defender nameplate on the vehicle in your photo is probably not original if it was built in 1989.
If it's a 1989 Land Rover 90, it will probably have the 19J engine, which is not a bad engine. It's just underpowered compared to its successor, the 200TDI (which itself is feeble).
Without more details on what's under the hood and what condition the frame, bulkhead, body, and other mechanicals are in, there isn't much to inform an opinion on valuation. If you're satisfied that it's in good shape and runs well and you want it for the Defender look/feel/experience, then that price is much lower than many I've seen, a little more than some. If you want to drive it on the highway or, heaven help you, up a mountain pass on the highway, you will find it a frustrating experience. But if you're tooling around town, going off-road locally, and enjoying it as a hobby vehicle, it will be loads of fun. If you think that's worth $20K, then it sounds like you've found a decent deal.