In most cases in Hi Fi, newer is better. There are some exceptions, but, for instance, a $1000 CD new in 2005 will almost certainly sound much better than a $1000 CD new in 1990, and should sell for considerably more than its older rival on the 2nd hand market.
But tuners are a special case. Digital tuners (push-button) replaced analogue ones in the early 1980s and with them came the end of an era during which major Japanese and American manufacturers were creating ever-more sophisticated and powerful analogue tuners. The best of these rival and even exceed top quality CD players and turntable systems for exciting and realistic sound quality.
The most famous of these is probably the Sansui TU-X1, a gigantic tuner (larger than most A/V receivers) which many people regard as the best tuner ever made (not a view held by all). Recently a Sansui TU-X1 sold for over US$2500. Its new price was less than this, but, taking inflation into account, would actually be close to three times this price, so the buyer may still consider he got a bargain!
But age takes its toll on electronics, and you need to know what you are getting into, when buying a vintage tuner. Electrolytic capacitors, which are used throughout tuners and other hi-fi components, may deteriorate or fail over time, degrading the performance of the component.
For example, I found a magnificent Yamaha CT-7000 tuner, built in 1976, on eBay. This tuner sold in Australia for around $2000, which would be equivalent to over $6000 in today's money! It was in superb cosmetic condition but was hardly working at all. Even so, it sold for almost $400 - far more than most people would pay for a fully functioning tuner. And, I knew it would require substantial repair. I was prepared to pay to restore this legendary piece of hi-fi to its former glory. The cost of repair, which included replacement of over 50 capacitors, was more than I paid for the tuner!
In the end I got a truly superb tuner, which sounds incredible, for a fraction of its true value. But I bought with my eyes wide open.
When evaluating whether a tuner is worth buying, you need to decide whether you want something which will work out of the box, or whether you are willing to buy a "bargain" and sink enough money into it afterward to bring out its potential. If the latter, make sure that you start with a quality product. Check on the web, whether the tuner on offer is regarded as jewel or junk.
Then decide what your limit is for initial purchase, and how much you are prepared to add to that for restoration. Remember that good electronics repairers charge by the hour and may use expensive replacement components. Some special electrolytic capacitors can cost $20 each which can add up to a hefty bill.
Here's another important issue you may not be aware of:
Australia shares FM standards and AM standards with Europe, but these are not the same as those of the U.S. Even if the tuner can be switched from US power (110V/60Hz) to European (220V/50Hz) or Australian/U.K. power (240V/60Hz), that doesn't mean that a U.S. tuner will operate correctly in Australia. Two other settings need to change: The steps between AM stations in the U.S. are 10 KHz, while the rest of the world uses 9 KHz steps. Most AM/FM tuners have a simple switch (usually internal) to change this.
But FM may not be so simple. The FM band in the U.S. uses 75μSec de-emphasis vs 50μSec de-emphasis in Australia and Europe. This affects the high frequency reproduction. Playing Australian FM through a US tuner sounds dull and lifeless. Playing US FM through a European or Australian tuner sounds shrill and harsh. While some tuners have an internal switch for de-emphasis, many have no way to adjust the de-emphasis apart from making actual changes to the electronics (a costly and sometimes difficult option). So, when buying a tuner from overseas, it pays to do some research as to how easy it is to change to your local settings.
Good luck and choose well. At best, you will be rewarded with FM reproduction to die for!