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 75 (MBC), indicating that it is in relatively good shape. This suggests that its value should be above the average for coins with lower conditions.

2. Starting Price and Average Hammer Price: The starting price is 400 euros, and the average hammer price in previous auctions is 642.75 euros. This indicates that, generally, coins of this reference tend to sell for a price higher than the starting price.

3. Sales History: The coin has been auctioned 29 times, of which it has sold 28 times. This suggests high demand and good interest from collectors.

4. Previous Prices: Looking at previous hammer prices, there are examples of coins with similar conditions that have sold for prices that vary significantly. For instance, in 2023, a coin with a condition of 70 sold for 660 euros, which is a good indicator that a coin with a condition of 75 could reach a similar or higher price.

5. Recent Trends: In the most recent auctions, hammer prices have shown an increase, which could indicate an upward trend in the value of these coins.

Taking all these factors into account, an estimated maximum purchase price for the coin could be around 800-900 euros. This is based on the fact that, although the starting price is 400 euros, the auction history and condition suggest it could reach a significantly higher price at auction.