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 of 85 (MBC+/EBC-), indicating that it is in fairly good shape, although not the best possible condition.

2. Previous Auction Prices:
- The average hammer price in previous auctions is 1139.09 euros.
- In recent auctions, the hammer price has varied, with a maximum of 2000 euros in 2023 for a coin with a condition of 95.

3. Price Trends:
- Starting prices have been increasing over time, suggesting an upward trend in the coin's value.
- The last auction with a hammer price of 1200 euros for a condition of 70 and 1800 euros for a condition of 80 also indicates that prices are on the rise.

4. Current Starting Price: The starting price is 1100 euros, which is a good starting point for the auction.

Taking all these factors into account, an estimated maximum purchase price could be in the range of 1300 to 1500 euros. This is based on the current condition of the coin and previous hammer prices, as well as the trend of rising prices in the market.

Therefore, a reasonable maximum purchase price could be 1400 euros.