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 translates to a higher price compared to coins with lower conditions.

2. Auction History:
- The average hammer price for similar coins in a condition of 75 is approximately 22.89 euros.
- In previous auctions, the hammer price for coins with a condition of 75 has varied, but it has reached up to 65 euros in a previous auction with a condition of 80.

3. Starting Price and Previous Prices:
- The current starting price is 12 euros, which is relatively low compared to the average hammer price.
- In previous auctions, the hammer price has ranged from 5 to 65 euros, depending on the condition and other factors.

4. Sales Trends: The coin has been sold in 18 out of 22 auctions, indicating good demand.

Based on these factors, an estimated maximum purchase price could be set in a range between 30 and 40 euros, considering that the condition is good and the average hammer price is 22.89 euros. However, given the history of higher hammer prices, a maximum purchase price of 40 euros seems reasonable.