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 rating of 80 (MBC+). This is an important factor, as coins in better condition tend to reach higher prices.

2. Auction History:
- The average hammer price for similar coins has been 78.46 euros.
- In previous auctions, coins with a similar condition (around 80) have reached hammer prices that vary, but some examples are:
- 40 euros in an auction with a condition of 80.
- 90 euros in an auction with a condition of 80.
- 100 euros in an auction with a condition of 85.

3. Starting Price: The starting price is 20 euros, indicating that the auctioneer expects interest in the coin, but the starting price does not always reflect the final hammer price.

4. Price Trends: Observing the hammer prices of coins with similar conditions, it seems that the price range could be between 40 and 100 euros, depending on demand and other market factors.

Based on these points, an estimated maximum purchase price for this coin could be in the range of 80 to 100 euros. This is based on the historical hammer prices for coins of similar condition and the interest it might generate at the auction. However, it is important to keep in mind that the final price may vary depending on competition at the auction and other market factors.