To estimate a maximum purchase price for the coin in question, we will consider several factors:

1. Current Condition: The coin has a condition of 75 (MBC), which indicates that it is in fairly good shape, although it is not the best possible condition.

2. Auction History:
- The average hammer price in previous auctions is 420.85 euros.
- The condition of the coins auctioned in the past varied, but there are examples of coins with higher conditions that have sold for higher prices. For example, a coin with a condition of 90 sold for 450 euros.

3. Starting Price and Estimated Price:
- The current starting price is 350 euros, which is slightly higher than the average starting price of 319.23 euros.
- The estimated hammer price for similar coins has been up to 650 euros in the past, although this depends on the condition.

4. Sales Trends: Since this coin has sold every time it has been auctioned, there is a consistent demand.

Based on these factors, a reasonable maximum purchase price could be in the range of 400 to 450 euros, considering that the condition is good but not excellent, and that the average hammer price in the past has been 420.85 euros.

Therefore, an estimated maximum purchase price could be 450 euros.