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

1. Current Grade: The coin has a grade of 75 (MS), indicating that it is in fairly good condition, but not of the highest quality (PR).

2. Previous Auction Prices: By looking at previous auction prices for coins with a similar grade (75), we can see that they have varied. For example:
- In 2011, a coin with a grade of 75 sold for 473.14 euros.
- In 2016, another coin with a grade of 75 sold for 120 euros.
- In 2019, a coin with a grade of 80 sold for 340 euros.

3. Average Auction Price: The average auction price for this coin is 519.38 euros, suggesting that, in general, coins of this reference tend to reach higher prices.

4. Starting Price: The current starting price is 125 euros, which is significantly lower than the average auction price.

5. Recent Trends: In the most recent auctions, the auction prices have shown a trend of being higher, especially for coins in good condition.

Based on these factors, an estimated maximum purchase price for this coin could be in the range of 300 to 500 euros, considering that the grade is 75 and the average auction price is 519.38 euros. However, since the starting price is 125 euros, it is possible that buyers may be willing to pay a bit more, but not necessarily reaching the average auction price.

Therefore, a reasonable maximum purchase price could be approximately 400 euros.