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

1. Current Condition: The coin has a condition of 80 (MBC+), indicating that it is in relatively good shape. This suggests that its value could be in the upper range of previous auction prices.

2. Previous Auction Prices: By looking at previous auction prices for coins with similar conditions (around 80), we can see that prices have varied. For example:
- In 2020, a coin with a condition of 80 was auctioned for 575 euros.
- In 2018, another coin with the same condition was auctioned for 420 euros.
- In 2023, a coin with a condition of 85 was auctioned for 850 euros.

3. Starting Price and Average Auction Price: The current starting price is 800 euros, and the average auction price for similar coins is 638.89 euros. This suggests that the starting price is relatively high compared to the auction average.

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

5. Estimation Based on Historical Data: Given that the coin has been auctioned 39 times in the past and has had varying auction prices, a maximum purchase price could be positioned in a range that considers both the starting price and previous auction prices.

Taking all these factors into account, an estimated maximum purchase price for this coin could be approximately 850 to 900 euros. This is based on the current condition, previous auction prices, and the trend of increasing prices in recent auctions.