To estimate a maximum purchase price for the coin that will be auctioned, we can consider several factors based on the provided data:

1. Starting price: 800 euros.
2. Coin condition: 85 (MBC+/EBC-), which indicates good condition.
3. Auction price history: The average hammer price in previous auctions for coins of similar condition (85) has been approximately 610.65 euros.
4. Previous hammer prices: In past auctions, hammer prices for coins with a condition of 85 have varied, with some reaching up to 1400 euros under similar conditions.

Given that the starting price is 800 euros and considering that the condition is good, a hammer price in the range of 1000 to 1200 euros could be reasonable, based on the auction price history for coins of similar condition.

Therefore, an estimated maximum purchase price could be around 1200 euros. This allows for a margin for competition in the auction, given that the starting price is 800 euros and there is a price history suggesting it could reach higher figures.