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

1. Starting Price: 300 euros.
2. Condition: The coin has a condition rating of 85 (MBC+/EBC-), indicating that it is in good condition. This generally can increase its value compared to coins with lower conditions.
3. Auction History: The coin has been auctioned 2 times in the past and has sold on both occasions. This suggests that there is interest in the coin and that it has an active market.
4. Previous Prices: In the November 2020 auction, the coin had a starting price of 250 euros and a hammer price of 420 euros. This indicates that the hammer price can be significantly higher than the starting price, especially if the condition is good.

Given that the average hammer price for similar coins is 585 euros and considering the condition of this coin, an estimated maximum purchase price could be in a range between the previous average hammer price and the current starting price.

Taking all these factors into account, an estimated maximum purchase price could be around 500 to 600 euros. This is based on market interest, good condition, and the hammer price history.