To estimate a maximum purchase price for the coin that will be auctioned, we can analyze the provided data, especially previous auction prices and the average hammer price.

1. Starting price: 500 euros.
2. Average hammer price: 406.31 euros.
3. Previous hammer prices:
- There are several hammer prices in the range of 170 to 4200 euros, with a focus on the most recent and relevant prices.
- The highest hammer prices in the past have been significantly above the current starting price.

4. Condition: No information is provided about the condition of the current coin, which may affect its value. However, by observing the hammer prices of coins with similar conditions (75-90), we can infer a price range.

5. Trends: The overall trend shows that hammer prices can vary considerably, but the highest prices have been recorded for coins in good condition.

### Maximum Purchase Price Estimation:
Given that the starting price is 500 euros and considering that the average hammer price is 406.31 euros, a reasonable maximum purchase price could be slightly higher than the starting price, as buyers are often willing to pay more in competitive auctions.

Maximum purchase price estimate: 700 euros.

This price takes into account the possibility that the coin may have significant value in the market, especially if there is interest among collectors.