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

1. Current starting price: 300 euros.
2. Average starting price in the past: 387.5 euros.
3. Average hammer price in the past: 520 euros.
4. Current condition: 75 (MBC), which is quite a good condition.
5. Auction history: The coin has been auctioned 4 times, of which it has sold 3 times. This indicates a consistent interest in the coin.

Analyzing the previous auction prices:
- In 2016, the hammer price was 550 euros with a starting price of 400 euros.
- In another auction in 2016, the hammer price was 460 euros with a starting price of 350 euros.
- In 2024, the hammer price was 550 euros with a starting price of 500 euros and a condition of 80.

Given that the current condition of the coin is 75, which is lower than the 80 in the 2024 auction, it is reasonable to expect that the hammer price will not exceed 550 euros, but it could be in a similar range.

Considering the average hammer price of 520 euros and the price history, an estimated maximum purchase price could be positioned between the average hammer price and the highest observed hammer price.

Therefore, an estimated maximum purchase price could be approximately 500 euros. This is based on the current condition and the historical interest in the coin.