To estimate a maximum purchase price for the coin in question, we can analyze the provided data, especially the previous auction prices and the current condition of the coin.

1. Current Condition: The coin has a condition rating of 80 (MBC+). This indicates that it is in relatively good shape, which may positively influence its price.

2. Previous Auction Prices:
- The average auction price for similar coins has been 134.97 euros.
- Looking at the auction prices of coins with similar conditions (around 75-85), we can see that the auction prices have varied, but many have exceeded 100 euros.

3. Starting Price and Estimated Price:
- The starting price is 90 euros, which is a good starting point.
- The estimated price for this auction is 150 euros, suggesting that there are expectations for the coin to sell at a higher price.

4. Historical Trends:
- In previous auctions, coins with similar conditions have reached auction prices ranging from 100 to 200 euros, depending on demand and other factors.

Based on these points, an estimated maximum purchase price for this coin could be between 150 and 200 euros, considering that the condition is good and that interest in coins of this type may be high. Therefore, a reasonable maximum purchase price could be 200 euros.