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

1. Coin 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 14 times in the past, of which it has sold 11 times.
- The average starting price is 23.21 euros, and the average hammer price is 28.64 euros.
- In previous auctions, hammer prices for coins with similar condition (80) have varied, but there was a recent hammer price of 18 euros in 2021 and a hammer price of 80 euros in 2019.

3. Price Trends:
- Hammer prices for coins with a condition rating of 70 to 80 have ranged between 15 and 80 euros, suggesting a wide range depending on demand and supply at the time of the auction.

4. Current Starting Price: The starting price is 18 euros, which is relatively low compared to the average hammer price.

Based on this data, an estimated maximum purchase price could be set in a range between 30 and 50 euros, considering that the condition is good and the average hammer price is 28.64 euros.

Therefore, a reasonable maximum purchase price could be 40 euros. This allows for some margin for competition in the auction, given that the starting price is low and the condition is good.