To estimate a maximum purchase price for the coin in question, we can consider several factors:

1. Current Condition: The coin has a condition rating of 80 (MS+), indicating that it is in quite good shape. This generally translates to a higher price compared to coins with lower conditions.

2. Auction History:
- The average hammer price for similar coins has been 63.08 euros.
- In previous auctions, hammer prices have varied, but there are examples of hammer prices reaching up to 130 euros (in 2016) for coins of similar or lower condition.

3. Starting Price and Estimated Price:
- The current starting price is 75 euros, which is higher than the average starting price of 50.83 euros. This may indicate an expectation that the coin will sell for a higher price due to its good condition.
- The estimated price of 90 euros in a previous auction (2018) also suggests that there is potential to reach higher prices.

4. Sales Trends: Of the 18 times it has been auctioned, 12 have resulted in sales, indicating a consistent interest in this coin.

Taking all these factors into account, an estimated maximum purchase price could be in the range of 90 to 130 euros, considering that the condition is good and there are precedents for high hammer prices. However, to be conservative and taking into account the starting price, a reasonable maximum purchase price could be 100 euros. This allows for competition in the auction and reflects the interest in the coin.