To estimate a maximum purchase price for the coin in question, we can consider several factors based on the provided data:

1. Current starting price: 800 euros.
2. Condition of the coin: 85 (ebc-), indicating good condition, although not the best.
3. Auction history:
- It has been auctioned 5 times in the past, of which it has sold 3 times.
- The average starting price in previous auctions is 2045.3 euros, and the average hammer price is 3144.39 euros.
4. Previous prices:
- In 2012, it sold for 600 euros.
- In 2013, it sold for 6833.17 euros (condition 90).
- In 2013, it sold for 2000 euros (condition not specified).
- In 2015, a hammer price of 4334.7 euros was estimated (condition not specified).

Given that the current condition of the coin is 85, which is lower than 90 (which sold for 6833.17 euros), it is reasonable to expect that the hammer price will not reach that level. However, the average hammer price of 3144.39 euros suggests that there is significant potential for a higher selling price.

Taking all these factors into account, an estimated maximum purchase price could be in a range between the average hammer price and a bit more, considering the condition.

Therefore, an estimated maximum purchase price could be around 2500 to 3000 euros. This reflects a balance between the condition, the price history, and the current starting price.