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

1. Current Condition: The coin has a condition rating of 80 (MBC+), indicating 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 the past is 71.8 euros.
- The coin has had hammer prices that vary, with the highest being 260 euros in a previous auction with a condition of 85.
- The price trend shows that coins with a similar condition (80) have reached hammer prices of up to 135 euros.

3. Starting Price and Estimated Price:
- The current starting price is 75 euros, and the estimated price is 150 euros. This suggests that there are expectations for the coin to sell for a considerably higher price than the starting price.

4. Demand and Supply: The coin has been auctioned 31 times, of which 20 have resulted in sales. This indicates a consistent interest in the coin, which may influence the price.

Based on these factors, an estimated maximum purchase price could be in the range of 120 to 150 euros, considering that the condition is good and there is a history of hammer prices that supports this estimate. However, given the starting price of 75 euros, it is likely that buyers will be willing to bid more, especially if there is interest in the coin.

Therefore, a reasonable maximum purchase price could be 150 euros.