To estimate a maximum purchase price for the coin in question, we can analyze the provided data and consider several factors:

1. Current Condition: The coin has a condition of 75 (MBC), indicating that it is in quite good shape. This is important because coins in better condition tend to fetch higher prices at auctions.

2. Auction History:
- The coin has been auctioned 7 times, of which it has sold 6 times. This indicates a consistent interest in the coin.
- Previous hammer prices for coins with similar conditions (75 and 80) have varied, but generally, the hammer prices have been in a range that suggests the coin could reach significant prices.

3. Previous Hammer Prices:
- In 2020, a coin with a condition of 80 sold for 320 euros.
- In 2018, another with a condition of 75 sold for 345 euros.
- In 2013, one with a condition of 70 sold for 280 euros.
- In 2020, another with a condition of 80 sold for 350 euros.
- In 2023, a coin with a condition of 80 sold for 1005 euros, indicating a high potential in the market.

4. Starting Price and Average Hammer Price:
- The current starting price is 200 euros, and the average hammer price in previous auctions is 398.33 euros. This suggests that the hammer price could be considerably higher than the starting price.

Based on these factors, an estimated maximum purchase price could be in the range of 400 to 600 euros, considering that the condition of the coin is good and there is a history of hammer prices that supports this estimate.

Therefore, a reasonable maximum purchase price could be approximately 500 euros.