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 (MS+), indicating that it is in good condition. This generally increases its value compared to coins with lower conditions.

2. Auction History: The coin has been auctioned 3 times in the past and has sold on all occasions. This suggests that there is consistent interest in the coin, which can positively influence its price.

3. Previous Hammer Prices: The previous hammer prices are 360 and 400 euros for coins with a condition of 70. Since the current condition is better (80), it is reasonable to expect that the hammer price will be higher than these values.

4. Average Hammer Price: The average hammer price of 686.67 euros is an important indicator. Although this value may include coins of different conditions, it suggests that there is significant potential to reach higher prices.

5. Starting Price and Average Starting Price: The starting price is 300 euros, and the average starting price is 325 euros. This indicates that the starting price is relatively low compared to previous hammer prices.

Taking all these factors into account, an estimated maximum purchase price could be in the range of 500 to 700 euros, considering that the condition is better than previous auctions and that interest in the coin appears to be high. A more conservative price could be around 600 euros, while a more optimistic price could approach 700 euros.

Therefore, a reasonable estimated maximum purchase price could be 650 euros.