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+). This is an important factor, as coins in better condition tend to reach higher prices.

2. Previous Auction Prices: By observing previous auction prices, we can see that coins with similar conditions (80-85) have had auction prices that vary, but generally range between 400 and 850 euros. For example:
- In 2022, a coin with a condition of 80 was auctioned for 420 euros.
- In 2023, a coin with a condition of 95 was auctioned for 850 euros, indicating that condition has a significant impact on price.

3. Average Auction Price: The average auction price for this coin is 696.78 euros, which gives us a good reference for what buyers are willing to pay.

4. Starting Price: The starting price is 500 euros, indicating the starting point for the auction.

5. Auction Trends: The coin has been auctioned 24 times, of which 20 have sold. This suggests there is consistent interest in this coin.

Based on these factors, an estimated maximum purchase price could be in the range of 700 to 850 euros, considering that the condition is slightly lower than coins that have sold for higher prices, but still, the interest in the coin and the average auction price suggest it could reach a competitive price.

Therefore, an estimated maximum purchase price could be 800 euros.