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 (MBC), which indicates that it is in fairly good shape, although not the best possible condition.

2. Auction History:
- In the 2020 auction, a coin with a condition of 80 sold for 240 euros, with a starting price of 200 euros.
- In the 2018 auction, a coin with a condition of 70 sold for 160 euros, with a starting price of 90 euros.

3. Starting Price and Average Hammer Price:
- The current starting price is 220 euros, which is higher than the starting prices of previous auctions, but it also reflects an increase in demand or value for the coin.
- The average hammer price is 260 euros, suggesting that, in general, coins of this reference tend to sell above their starting price.

4. Price Trends: Given that the coin has had an average hammer price of 260 euros and considering that the current condition is slightly lower than that of the 2020 auction, we could estimate that the hammer price might be in a similar range, but adjusted for condition.

Taking all these factors into account, an estimated maximum purchase price could be around 240-260 euros, considering that the condition is slightly lower than that of the 2020 auction, but still in fairly good shape.

Therefore, a reasonable maximum purchase price could be 250 euros.