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 fairly good shape, although not the best possible condition.

2. Auction History:
- In the past, the coin has been auctioned 5 times and has sold on all occasions, indicating a consistent interest in the coin.
- Previous hammer prices for coins with a similar condition (75) have been 324, 301, and 820 euros. This suggests that the hammer price can vary significantly depending on demand and other factors.

3. Average Hammer Price: The average hammer price for this coin is 751.2 euros, providing a good reference for the expected value.

4. Current Starting Price and Average Starting Price: The current starting price is 300 euros, which is below the average starting price of 420 euros. This could indicate that there is a buying opportunity at a lower price than average.

5. Recent Trends: In the most recent auction, a coin with a condition of 85 sold for 1200 euros, indicating that there is an active market and that prices can be high for coins in better condition.

Taking all these factors into account, an estimated maximum purchase price for the coin with a condition of 75 could be in the range of 700 to 800 euros, as this aligns with the average hammer price and historical interest in the coin. However, since the starting price is 300 euros, it may be possible to acquire it for a price below this range, depending on the competition in the auction.

Therefore, an estimated maximum purchase price could be 800 euros.