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 price could be in the upper range of previous auctions.

2. Previous Prices: By looking at previous auction prices for coins with similar conditions (70-80), we can see that auction prices have varied, but there are examples where prices have reached up to 120 euros for coins in condition 80.

3. Average Hammer Price: The average hammer price for this coin is 70.75 euros, which can also serve as a reference.

4. Auction Trends: The coin has been auctioned 28 times, of which 23 have been sold. This indicates a consistent interest in the coin, which may increase its value.

5. Starting Price: The starting price is 30 euros, which is relatively low compared to previous hammer prices.

Based on these factors, an estimated maximum purchase price could be in the range of 80 to 100 euros, considering that the condition is good and there are precedents of higher hammer prices for similar conditions.

Therefore, a reasonable estimated maximum purchase price could be 90 euros.