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

1. Current Condition: The coin has a condition rating of 85 (ebc-), which is relatively high and may positively influence its value.

2. Previous Prices: By observing previous auction prices, we can see that in recent auctions, coins with similar conditions have reached hammer prices of up to 1400 euros (as seen in the auction on April 27, 2022).

3. Starting Price and Average Hammer Price: The current starting price is 900 euros, and the average hammer price in previous auctions is 656.31 euros. This suggests that the starting price is higher than the average hammer price, which could indicate an expectation of greater interest in this auction.

4. Sales Trends: The coin has been auctioned 23 times, of which it has sold 16 times. This indicates a reasonable sales rate, which can be a good indicator of demand.

5. Previous Estimates: Estimated prices in previous auctions have varied, but have reached up to 2000 euros on some occasions, suggesting there is potential for a high hammer price.

Taking all these factors into account, an estimated maximum purchase price could be in the range of 1200 to 1500 euros, considering the current condition and previous hammer prices. This would allow for a competitive bid without exceeding the market value observed in past auctions.