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 85 (ebc-), indicating that it is in fairly good shape, but not the best possible condition.

2. Auction History:
- The average hammer price for similar coins has been 91.9 euros.
- In previous auctions, hammer prices have varied, but there are examples of sales that reached up to 320 euros in exceptional conditions.
- Most hammer prices for coins in similar conditions (85) have been in the range of 38 to 110 euros.

3. Starting Price: The current starting price is 15 euros, which is relatively low compared to previous hammer prices.

4. Sales Trends: The coin has been sold in 11 out of the last 13 auctions, indicating a consistent interest in it.

Based on these factors, an estimated maximum purchase price could be set in a range that reflects both the condition and the price history. Given that the condition is good but not excellent, and considering the average hammer price, a reasonable maximum purchase price could be:

Estimated maximum purchase price: 100 euros.

This price reflects a balance between the current condition and the hammer price history, taking into account that there may be competing bidders interested in the auction.