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 fairly good shape, although not the best possible condition.

2. Auction History:
- It has been auctioned 6 times in the past, of which it has sold 5 times. This indicates good demand.
- The average hammer price in previous auctions is 668 euros, suggesting that, in general, buyers are willing to pay a considerably higher price than the starting price.

3. Previous Prices:
- In past auctions, hammer prices have varied, but there are examples of auctions that reached up to 1200 euros in similar conditions (85).
- The lowest hammer price in previous auctions was 360 euros, indicating that even in lower conditions, the coin has significant value.

4. Starting Price and Estimates:
- The current starting price is 350 euros, which is reasonable given the price history.
- The estimated hammer price for this auction is 400 euros, which is also a good indicator.

Based on these factors, an estimated maximum purchase price could be in the range of 700 to 800 euros, considering that the average hammer price has been 668 euros and that demand appears to be strong. This also takes into account that the current condition is slightly lower than the higher conditions that have reached higher prices in the past.

Therefore, a reasonable maximum purchase price could be 800 euros.