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

1. Starting price: 350 euros.
2. Current grade: 75 (MS), which is a fairly good grade, although not the best.
3. Auction history:
- In the auction on April 24, 2024, a coin with a grade of 70 sold for 550 euros, with a starting price of 200 euros.
- In the auction on June 25, 2024, a coin with a grade of 95 sold for 1000 euros, with a starting price of 400 euros.

4. Average hammer price: 646.67 euros, indicating that, in general, coins of this reference tend to sell for relatively high prices.

Given that the grade of the coin to be auctioned is 75, which is better than the 70 (550 euros) but lower than the 95 (1000 euros), we can estimate that the hammer price could be in an intermediate range.

Considering the average hammer price and the price history, an estimated maximum purchase price could be between 700 and 800 euros. This is based on the price trends of coins with similar grades and the fact that the coin in question has a starting price of 350 euros, suggesting that there could be significant interest.

Therefore, an estimated maximum purchase price could be 750 euros.