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), indicating that it is in quite good shape. This generally translates to a higher price compared to coins in lower condition.

2. Auction History:
- The average hammer price in previous auctions has been 70 euros.
- The coin has been auctioned 6 times, of which it has sold 5 times, indicating good demand.
- Previous hammer prices have varied, but the highest was 75 euros on several occasions.

3. Starting Price and Average Starting Price:
- The current starting price is 30 euros, which is relatively low compared to the average starting price of 50 euros and the average hammer price of 70 euros.

4. Price Trends:
- In previous auctions, hammer prices have ranged from 60 to 75 euros, with an estimated price of up to 100 euros in a previous auction.

Given that the coin's condition is 75 and considering the price history, an estimated maximum purchase price could be in the range of 70 to 90 euros. This is based on the average hammer price and the fact that the condition is good.

Therefore, a reasonable estimated maximum purchase price for this coin could be 80 euros. This allows for a margin for competition in the auction, given that the starting price is low and there is interest in the coin.