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 (MS+), indicating that it is in relatively 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 45.8 euros.
- The condition of coins auctioned in the past varies, but there are examples of coins with similar condition (80) that have sold for prices ranging from 14 to 85 euros.

3. Recent Trends:
- In the most recent auctions, hammer prices for coins with a condition of 80 have been in the range of 35 to 60 euros.
- The estimated price for the upcoming auction is 50 euros, suggesting an expectation that the coin will sell for a reasonable price.

4. Starting Price: The starting price is 20 euros, which may attract more bidders, but it could also indicate that the seller expects a lower price initially.

Based on these factors, an estimated maximum purchase price could be in the range of 60 to 80 euros, considering that the condition is good and there is a price history supporting this estimate. However, since the market can be volatile and depends on demand at the time of the auction, a more conservative price might be around 50 to 60 euros.

Therefore, a reasonable estimated maximum purchase price could be 70 euros.