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 in question has a condition rating of 100 (S/C), indicating that it is in very good condition. This generally translates to a higher price compared to coins with lower conditions.

2. Starting price: The starting price is 400 euros. This is the starting point for the auction.

3. Historical data:
- The average hammer price for similar coins in the past is 252.42 euros.
- Previous auctions of this same coin have shown hammer prices that varied, but generally, the hammer prices have been in a range that can reach up to 420 euros under similar conditions.

4. Auction trends: The coin has been auctioned 82 times, of which 77 have been sold. This indicates good demand and sustained interest in the coin.

5. Previous hammer prices: The highest hammer prices for coins in similar condition have reached up to 420 euros, suggesting that there is potential for this coin to reach a similar or even higher price, given its condition.

Based on these factors, an estimated maximum purchase price could be in a range between 400 and 500 euros, considering that the starting price is 400 euros and that the condition is excellent. A price of 450 euros could be a reasonable target, as it falls in the middle of this range and reflects both the interest in the coin and its condition.