To estimate a maximum purchase price for the coin in question, we will consider several factors:

1. Current condition: The coin has a condition of 75 (MBC), which indicates that it is in quite good shape. This generally increases its value compared to coins in lower condition.

2. Auction history:
- In the 2017 auction, the coin started at 18 euros and was sold for 20 euros.
- In the 2020 auction, it started at 12 euros and was sold for 12 euros.

3. Average hammer price: The average hammer price from previous auctions is 16 euros, which can serve as a reference.

4. Current starting price: The starting price is 15 euros, which is slightly lower than the average hammer price from previous auctions.

Given that the condition of the coin is good and considering the price history, an estimated maximum purchase price could be set between the average hammer price and a bit higher, since the condition is better than in some of the previous auctions.

Therefore, an estimated maximum purchase price could be approximately 25 euros. This is based on the fact that the condition is good and that interest in the coin could lead to a higher hammer price than average.