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

1. Current Condition: The coin has a condition of 75 (VF), indicating that it is in fairly good shape, although not the best possible condition.

2. Previous Prices:
- In the December 2019 auction, the coin was auctioned with a starting price of 2500 euros and sold for 6500 euros, with a condition of 80.
- In May 2017, the coin was auctioned with a starting price of 4500 euros and sold for 4500 euros, although the condition is not specified.

3. Average Starting and Final Prices: The average starting price is 3000 euros and the average final price is 4500 euros. This suggests that, in general, coins of this reference tend to reach final prices that are significantly higher than the starting price.

4. Auction Trends: Since the coin has sold every time it has been auctioned, this indicates a consistent interest in the coin, which may influence its price.

Based on these factors, we can make an estimation. Given that the current condition is 75 and considering that in the past final prices of up to 6500 euros have been reached for higher conditions, a reasonable maximum purchase price could be between the average final price (4500 euros) and the highest final price (6500 euros).

Therefore, an estimated maximum purchase price could be around 5500 euros. This reflects a balance between the current condition and historical prices, taking into account that the coin is not in the best possible condition.