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

1. Coin Condition: The coin to be auctioned has a condition of 80 (MBC+). This is an important factor, as coins in better condition tend to reach higher prices.

2. Auction History:
- The average hammer price for similar coins in condition 80 has been 480 euros (in the 2021 auction) and 800 euros (in the 2022 auction).
- In recent auctions, the hammer price has varied, but it has reached up to 1200 euros for a coin with a condition of 85.

3. Starting Price: The starting price of the coin is 350 euros, which may influence bidder interest.

4. Price Trends: The average hammer price for similar coins has been significantly higher than the starting price, suggesting there is potential for an increase in the final price.

5. Analysis of Previous Prices:
- The coin has had a hammer price of 800 euros in a previous auction with the same condition (80).
- The trend shows that, although some coins did not sell, those that did reached high prices.

Based on these factors, an estimated maximum purchase price could be in the range of 600 to 800 euros, considering that the highest hammer price for a similar condition was 800 euros and that the average hammer price has been 617.25 euros.

Therefore, a reasonable estimated maximum purchase price could be 750 euros.