To estimate a maximum purchase price for the coin that will be auctioned, we can consider several factors based on the provided data:

1. Coin Condition: The coin has a condition rating of 85 (ebc-), which indicates that it is in quite good shape. This generally translates to a higher price compared to coins with lower conditions.

2. Starting Price: The starting price is 300 euros. This is the baseline for the auction.

3. Price History: By looking at previous auction prices for coins with a similar condition (85), we can see that the hammer prices have varied, but generally, they have been in a range that can help establish an estimated price. For example:
- In the auction on 2019-03-14, the hammer price was 260 euros.
- In the auction on 2023-03-16, the hammer price was 340 euros.

4. Average Hammer Price: The average hammer price for this coin is 261.57 euros, which is also a useful indicator.

5. Recent Trends: In the most recent auctions, hammer prices have shown a trend of increasing, especially for coins in good condition.

Given all this, an estimated maximum purchase price could be set in a range above the starting price, considering the good condition and the price history.

Final Estimate: A reasonable maximum purchase price could be approximately 350 to 400 euros. This is based on the coin's condition, the starting price, and the hammer price history.