To estimate a maximum purchase price for the coin in question, we can analyze the provided data, especially the current condition of the coin (75, MS) and previous auction prices.

1. Condition: The coin has a condition of 75 (MS), indicating that it is in quite good shape. This suggests that its value should be in the upper range of previous auction prices.

2. Previous auction prices: By looking at the auction prices of coins with similar conditions (70 and 75), we can see that:
- In the auction on 2023-03-22, a coin with a condition of 75 sold for 20 euros.
- In other auctions, coins with a condition of 70 sold for between 15 and 17 euros.

3. Price trend: The average auction price for this coin is 16 euros, and the average starting price is 13.75 euros. However, since the condition of the coin in question is better than most of the previous ones (75 versus 70), we can expect the auction price to be higher.

4. Starting price: The starting price is 20 euros, indicating that the seller has expectations of a higher auction price.

Based on these factors, an estimated maximum purchase price could be in the range of 20 to 25 euros, considering that the condition is good and that the highest previous auction price for a similar condition was 20 euros. Therefore, a reasonable maximum purchase price could be 25 euros.