To estimate a maximum purchase price for the coin in question, let's consider several factors:

1. Starting Price: 350 euros. This is the initial price set for the auction.
2. Condition: The coin has a condition rating of 85 (ebc-), indicating that it is in quite good shape. This generally increases its value compared to coins with lower conditions.
3. Auction History: The coin has been auctioned 3 times in the past and has sold on all occasions. This suggests that there is demand for this coin.
4. Previous Prices:
- In the October 2022 auction, the hammer price was 300 euros with a condition of 70.
- In December 2022, the starting price was 250 euros, but it did not sell, which may indicate that the starting price was too high for that condition.

5. Average Hammer Price: 400 euros, which is a good indicator of what buyers are willing to pay for coins in similar conditions.

Given that the condition of the current coin is significantly better (85) than the previous ones (70), and considering the average hammer price of 400 euros, we can estimate that the maximum purchase price could be in a range above the starting price.

Taking all these factors into account, an estimated maximum purchase price could be between 400 and 450 euros, given that the condition is better and there is a history of successful sales. However, the starting price of 350 euros also suggests that there might be an opportunity to buy at a lower price if competition is not high.

Therefore, a reasonable maximum purchase price could be 450 euros.