To estimate a maximum purchase price for the coin in question, we will consider several factors:

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

2. Starting Price: The starting price is 400 euros. This establishes a baseline for the auction.

3. Sales History: By observing previous auction prices for coins with similar conditions, we can see that prices have varied. For example, in recent auctions, coins with conditions ranging from 80 to 90 have reached hammer prices between 300 and 400 euros.

4. Recent Trends: In the most recent auctions, hammer prices for coins with similar conditions have been in the range of 360 to 480 euros, depending on demand and the rarity of the coin.

5. Average Hammer Price: The average hammer price for similar coins is 283.35 euros, but this may not reflect the current market value, especially if there is growing interest in the coin.

Based on these factors, an estimated maximum purchase price could be between 450 and 500 euros, considering that the coin is in good condition and the starting price is 400 euros. This also takes into account the possibility of competition in the auction, which could drive up the final price.