To estimate a maximum purchase price for the coin in question, let's consider several factors:

1. Starting Price: 6000 euros.
2. Condition: The coin is in a condition of 100 (S/C), indicating that it is in excellent shape. This generally increases its value.
3. Auction History:
- It has been auctioned 3 times and sold on all occasions.
- The average hammer price in previous auctions is 6416.67 euros.
- In the 2022 auction, the hammer price was 7750 euros, which is notably high compared to other hammer prices.
4. Price Trends: The estimated price for the next auction is 7000 euros, suggesting an expectation that the coin's value will remain stable or increase.

Given that the coin is in excellent condition and has a history of successful sales, the hammer price could be above the starting price. Considering the average hammer price and the highest hammer price in the past, a reasonable maximum purchase price could be in the range of 7500 to 8000 euros.

Therefore, an estimated maximum purchase price could be 8000 euros.