To estimate a maximum purchase price for the coin in question, let's consider the following factors:

1. Starting price: 100 euros.
2. Current condition: 90 (EBC), indicating good condition.
3. Auction history: The coin has been auctioned once before, with a hammer price of 120 euros and a starting price of 100 euros.
4. Average hammer price: 120 euros, which is the average price it has sold for in the past.
5. Auction expectations: It will be auctioned again in the coming days, which may influence demand.

Since the current condition is better than that of the previous auction (90 vs. 75), and considering that the average hammer price is 120 euros, we could estimate that the maximum purchase price could be in a range above the previous hammer price.

Taking all these factors into account, an estimated maximum purchase price could be approximately 130-150 euros. This reflects the good condition and potential interest in the coin, as well as the price history.

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 rating of 85 (ebc-), indicating that it is in quite good shape. This generally translates to a higher price compared to coins with lower conditions.

2. Starting Price: The starting price is 2100 euros, which sets a baseline for offers.

3. Sales History:
- The average hammer price in previous auctions is 1328.4 euros.
- In recent auctions, hammer prices for coins with similar conditions (around 85) have varied, with some reaching up to 1700 euros or more.

4. Recent Trends: In the most recent auctions, hammer prices have shown an increase, with some reaching up to 1834 euros. This suggests an upward trend in the value of this coin.

5. Comparison with Previous Prices: Looking at previous hammer prices for coins with similar conditions, the range has been approximately 1300 to 1700 euros in recent years.

Based on these factors, an estimated maximum purchase price could be in the range of 1700 to 2000 euros, considering that the starting price is 2100 euros and that the condition is good. However, since the starting price is higher than previous hammer prices, it is possible that buyers may be willing to offer a bit more, but not necessarily reaching the starting price.

Therefore, a reasonable maximum purchase price could be 1800 euros.