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

1. Coin Condition: The coin has a condition rating of 80 (MBC+), indicating that it is in relatively good shape. This suggests that its value should be above that of coins with lower conditions.

2. Previous Prices: By looking at previous auction prices for coins with similar conditions (80 and 85), we can see that auction prices have varied. For example:
- In 2015, a coin with a condition of 80 was auctioned for 230.11 euros.
- In 2022, a coin with a condition of 80 was auctioned for 190 euros.

3. Average Auction Price: The average auction price for this coin is 165.49 euros, providing an additional reference point.

4. Current Starting Price: The starting price is 400 euros, which is significantly higher than previous auction prices. This may indicate that the seller has high expectations, but it could also suggest that the market is willing to pay more for this particular coin.

5. Market Trends: Given that the coin has been auctioned 20 times in the past and has a 70% sell-through rate, this suggests there is consistent interest in this coin.

Based on these factors, an estimated maximum purchase price could be in the range of 200 to 300 euros, considering that this is a more realistic price compared to the starting price of 400 euros. This also takes into account the auction price history and the coin's condition.

Therefore, an estimated maximum purchase price could be 250 euros.