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

1. Current Condition: The coin has a condition rating of 90 (EF), indicating that it is in quite good shape. This generally translates to a higher price compared to coins in lower condition.

2. Auction History: The coin has been auctioned 12 times and has sold on all occasions. This suggests that there is consistent interest in the coin, which can increase its value.

3. Previous Hammer Prices: By looking at previous hammer prices, we can see that in recent auctions, especially in 2022, the coin reached hammer prices of up to 650 euros. This is an indicator that the market is willing to pay high prices for coins in good condition.

4. Starting Price and Average Hammer Price: The current starting price is 400 euros, and the average hammer price in the past has been 344.25 euros. However, given that the current condition is 90, which is quite high, it is reasonable to expect that the hammer price will be above this average.

5. Recent Trends: In the most recent auctions, hammer prices have been quite high, suggesting that interest in this coin has increased.

Taking all these factors into account, an estimated maximum purchase price could be in the range of 600 to 700 euros, given the history of hammer prices and the good condition of the coin. This would allow for competitive bidding in the auction without exceeding the market value observed in previous auctions.