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 75 (MBC), indicating that it is in acceptable shape, but not in the best condition.

2. Auction History:
- The average starting price in previous auctions is 937.46 euros, and the average hammer price is 1220.84 euros.
- In the past, hammer prices have varied significantly, ranging from 240 euros to 4019.51 euros, depending on the condition and demand at that time.

3. Price Trends:
- The last auction with a similar condition (75) had a hammer price of 276.35 euros, which may be a good indicator of the current market value.

4. Current Starting Price: The starting price is 640 euros, suggesting that the seller has expectations of a higher price, but it may also indicate that the market is not willing to pay that much.

Based on these factors, an estimated maximum purchase price could be positioned within a range between the average hammer price for similar conditions and the current starting price.

Given that the average hammer price for condition 75 has been approximately 276.35 euros in the last auction, and considering that the starting price is 640 euros, a reasonable maximum purchase price could be:

Estimated maximum purchase price: 800 - 1000 euros.

This range allows for a competitive offer without exceeding the historical value of the coin in similar conditions.