To estimate a maximum purchase price for the coin in question, we can analyze the provided data, especially the previous auction prices and the coin's condition.

1. Current Condition: The coin has a condition of 90 (EF), indicating that it is in quite good shape.

2. Previous Auction Prices:
- In previous auctions with a condition of 90, the hammer prices have been:
- 180 euros (2020-12-17)
- 170 euros (2019-04-25)
- 160 euros (2023-03-16)

3. Price Trends:
- The average hammer price for this coin is 87.93 euros, but the highest hammer prices for similar conditions are significantly higher.

4. Starting Price: The starting price is 90 euros, suggesting that the seller has expectations of a higher hammer price.

5. Competition Analysis: Given that there have been 17 previous auctions and 15 sales, demand seems to be good, which may influence a higher hammer price.

Based on these factors, an estimated maximum purchase price could be in the range of 160 to 180 euros, considering that in previous auctions with the same condition those prices have been reached. Therefore, a reasonable maximum purchase price could be:

Estimated maximum purchase price: 180 euros.