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

1. Current Condition: The coin has a condition of 85 (MS+/EF-), indicating that it is in quite good shape. This generally translates to a higher price compared to coins in lower conditions.

2. Historical Prices: By looking at previous auction prices, we can see that the coin has reached auction prices of up to 3100 euros in similar conditions (condition of 95) and has had lower auctions in inferior condition.

3. Starting Price and Average Price: The current starting price is 1800 euros, which is above the average starting price of 1059.38 euros and the average auction price of 1320.67 euros. This suggests that the seller has high expectations for this auction.

4. Recent Trends: In the most recent auction, the coin was estimated at 1800 euros and sold for 1950 euros, indicating that there is growing interest in this coin.

5. Comparison with Previous Prices: Most previous auction prices are in the range of 800 to 3100 euros, depending on the condition. Given that the current condition is 85, it is reasonable to expect that the auction price will be in the upper range of previous prices.

Taking all these factors into account, an estimated maximum purchase price could be around 2000-2200 euros. This is based on the current condition, recent interest, and previous auction prices. However, it is important to remember that the final price may vary depending on competition in the auction and demand at that time.