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 75 (MBC), indicating a medium state of preservation. This can influence the price, as coins in better condition tend to fetch higher prices.

2. Starting Price and Average Hammer Price: The current starting price is 900 euros, and the average hammer price in previous auctions is 1715.63 euros. This suggests that, in general, coins of this reference tend to sell for a significantly higher price than their starting price.

3. Sales History: The coin has been auctioned 29 times, of which it has sold 25 times. This indicates good demand and liquidity in the market for this coin.

4. Previous Prices: Looking at previous hammer prices for coins with a similar condition (75), the highest recorded price was 1300 euros in 2019. However, there are also higher hammer prices for coins in better condition.

5. Recent Trends: In the most recent auctions, hammer prices have been in the range of 1700 to 1800 euros for coins with conditions of 80 to 90. This suggests that the market is willing to pay more for coins in better condition.

Given all this, an estimated maximum purchase price for the coin in question could be in the range of 1300 to 1500 euros, considering its condition and price history. This would allow for a competitive offer without exceeding the market value observed in previous auctions.