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

1. Current Condition: The coin has a condition rating of 85 (EF-), indicating that it is in fairly good shape, but not the best possible condition.

2. Previous Prices: By looking at previous auction prices for coins with a similar condition (85), we can see that auction prices have varied, but generally remained in a range of 300 to 358 euros in recent auctions.

3. Starting Price and Average Hammer Price: The current starting price is 400 euros, which is higher than the average hammer price of 283.35 euros. This suggests that the seller expects a higher price, but it may also indicate that the market is a bit cooler.

4. Recent Trends: In the most recent auctions, hammer prices for coins of similar condition have been in the range of 350 to 400 euros, suggesting that there is interest in this price range.

5. Sales Expectations: Given that the coin has been auctioned 270 times in the past and has had a sell-through rate of 94.4% (255 out of 270), this indicates that there is an active market for this coin.

Based on these factors, an estimated maximum purchase price for this coin could be around 360-400 euros, considering that the starting price is 400 euros and that interest in the coin may lead to a hammer price close to that amount. However, it is important to keep in mind that the final price will depend on the competition in the auction and the interest of collectors at that time.