To estimate a maximum purchase price for the coin that will be auctioned, we will consider several factors:

1. Condition of the coin: The coin has a condition rating of 85 (MBC+/EBC-), which indicates that it is in fairly good shape, but not the best possible condition.

2. Starting price: The starting price is 400 euros.

3. Price history: By looking at the hammer prices from previous auctions for coins with a similar condition (85), we can see that the hammer prices have varied, but generally have remained in a range of 300 to 400 euros, with some cases reaching up to 450 euros.

4. Recent trends: In the most recent auctions, the hammer prices for coins of similar condition have been in the range of 350 to 400 euros, with some exceptional cases exceeding 400 euros.

5. Supply and demand: Given that there have been 270 previous auctions and 255 sales, demand seems to be quite high, which may influence a higher hammer price.

Based on these factors, an estimated maximum purchase price could be around 450 euros, considering that the coin is in good condition and there is significant interest in this type of coin. However, it is important to keep in mind that the final price may vary depending on the competition in the auction and other market factors at the time of the auction.