To estimate a maximum purchase price for the coin in question, we can analyze the provided data:

1. Starting price: 75 euros.
2. Coin condition: 70 (MBC-), indicating an average condition.
3. Average hammer price in previous auctions: 103.6 euros.
4. Price history:
- In 2018, with a condition of 70, it was sold for 60 euros.
- In 2018, with a condition of 80, it was sold for 128 euros.
- In 2014, there is no information on condition, but it was sold for 60 euros.
- In 2020, there is no information on condition, but it was sold for 110 euros.

Given that the condition of the current coin is 70, which is similar to the first auction in 2018 where it sold for 60 euros, and considering that the average hammer price is 103.6 euros, we can infer that the hammer price could be in a range between 60 and 110 euros.

However, since the average hammer price is significantly higher and the coin has a starting price of 75 euros, an estimated maximum purchase price could be around 100 euros, taking into account that collector interest and demand may influence the final price.

Therefore, an estimated maximum purchase price could be 100 euros.