To estimate a maximum purchase price for the coin that will be auctioned, we will consider several factors, including the starting price, the auction price history of similar coins, and the lack of information regarding the condition of the coin in question.

1. Starting Price: 150 euros.
2. Auction Price History:
- In previous auctions, the hammer prices have varied significantly. For example, in recent auctions, hammer prices have been seen at 60, 190, 220, and even 400 euros for coins with varying conditions.
- The general trend shows that hammer prices can be significantly higher than the starting price, especially for coins that have a good sales history.

3. Condition: The lack of information about the condition of this coin can be a risk factor. However, if we consider that the coin has a potential condition that could be acceptable, we might estimate a higher price.

4. Analysis of Previous Prices:
- The highest hammer prices in the past for similar coins have reached up to 650 euros, indicating that there is a market willing to pay high prices for quality coins.

Based on these factors, an estimated maximum purchase price could be around 300 euros. This price reflects a balance between the starting price, the auction price history, and the uncertainty regarding the condition of the coin. However, it is important to keep in mind that the final price will depend on the competition in the auction and the bidders' perception of the coin's value.