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 rating of 80 (MBC+), indicating that it is in relatively good shape. This can positively influence its price.

2. Current Starting Price and Previous Prices: The current starting price is 400 euros. In previous auctions, hammer prices have varied, but there are examples of hammer prices exceeding 300 euros, even reaching 860 euros in one case.

3. Price Trends: The average hammer price in previous auctions is 337.06 euros, suggesting that, in general, coins of this reference tend to sell for above their starting price.

4. Sales History: Out of the 26 times it has been auctioned, 19 have resulted in sales, indicating good demand for this coin.

5. Condition of Previous Sales: Coins with similar conditions (80 and 85) have had hammer prices ranging from 270 to 420 euros in recent auctions.

Taking all these factors into account, an estimated maximum purchase price could be in the range of 400 to 500 euros, considering that the starting price is 400 euros and there are precedents of higher hammer prices in similar conditions.

Therefore, a reasonable maximum purchase price could be 450 euros.