To estimate a maximum purchase price for the coin in question, we can analyze the provided data:

1. Current Condition: The coin has a condition rating of 80 (MBC+), indicating that it is in quite good shape.

2. Auction History:
- It has been auctioned 10 times in the past, of which it has sold 9 times.
- The average starting price in previous auctions is 27.86 euros, and the average hammer price is 35.13 euros.
- In past auctions, hammer prices have varied, with a maximum price of 75 euros in 2020 and other hammer prices around 20-50 euros on different occasions.

3. Price Trends:
- The last auction of this coin (2024-07-11) had a hammer price of 35 euros with a condition of 80, which is relevant for our estimation.
- In previous auctions, coins with similar conditions (75-90) have reached hammer prices of up to 50 euros.

4. Current Starting Price: The starting price is 18 euros, suggesting that the seller expects a lower price than the historical average.

Based on this data, an estimated maximum purchase price could be in the range of 35 to 50 euros, considering that the condition is good and that the price history shows that coins in similar conditions have reached those values.

Therefore, a reasonable estimated maximum purchase price could be 45 euros.