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

1. Current Condition: The coin has a condition rating of 80 (MBC+), indicating that it is in fairly good shape, but not the best possible condition.

2. Previous Auction Prices:
- The average hammer price in previous auctions is 2373.64 euros.
- The hammer prices of coins with similar conditions (80 and 85) have varied, but they have generally sold between 2200 and 2900 euros.

3. Current Starting Price: The starting price is 1800 euros, which is lower than the average hammer price in previous auctions.

4. Sales Trends: The coin has been auctioned 12 times, of which 11 have sold, indicating good demand.

5. Upcoming Auctions: There is an auction scheduled in the coming days, which may influence competition and interest in the coin.

Based on these factors, an estimated maximum purchase price could be in the range of 2500 to 2800 euros, considering that the condition is good and that the average hammer price in the past has been higher than the current starting price. This also takes into account the possibility that competition in the auction could drive up the final price.

Therefore, a reasonable maximum purchase price could be 2700 euros.