The watch appears to be genuine, original, and in good, to very good overall condition. Black dials are notoriously difficult to photograph well, so this one is difficult to assess.
Broadly speaking it is not a watch that most collectors would seeks, as the case, and lugs in particular, are not of a currently fashionable style. But assuming that you like the watch, and would be buying it wear, with few if any concerns about re-sale value, that should not be so important.
I would not ever take auction estimates seriously, and especially those at Catawiki, where the knowledge of their "experts" is, to put it kindly, often underwhelming. That said, because of the current high value of gold, the watch is probably worth at least 800€, and could certainly bring more if two interested parties were to bid in an auction setting.
One final note. While vintage Longines are widely held in high regard by collectors, models such as this one, which are not of the fashionable style, do not carry premiums related to the brand. In other words, a fashionable vintage Longines will typically be valued more (or much more) highly than similar watches produced by other manufacturers. But styles that are not widely sought typically cost around the same as those produced by similar quality manufacturers.
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