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 quite good shape. This generally translates to a higher price compared to coins with lower conditions.

2. Previous Prices: By looking at previous auction prices for coins with similar conditions (80 and 75), we can see that they have reached auction prices of up to 320 euros (in the case of the auction on 2022-07-07) and 270 euros (in the case of the auction on 2023-02-28).

3. Average Hammer Price: The average hammer price for this coin is 138.67 euros, which can also serve as a reference.

4. Starting Price and Average Starting Price: The current starting price is 70 euros, and the average starting price is 82.1 euros. This suggests that the starting price is reasonable compared to the auction history.

5. Sales Trends: The coin has been auctioned 20 times, of which 15 have sold, indicating good demand.

Based on these factors, an estimated maximum purchase price could be in the range of 200 to 300 euros, considering that the condition is good and there are precedents for high hammer prices in the past.

Therefore, an estimated maximum purchase price could be 250 euros. This is based on the trend of previous hammer prices and the current condition of the coin.