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 a medium-low quality. This may influence the price, as coins in better condition tend to fetch higher prices.

2. Starting Price and Average Hammer Price: The starting price is 40 euros, and the average hammer price in previous auctions is 81.9 euros. This suggests that, generally, coins of this reference tend to sell for a price higher than the starting price.

3. Sales History: The coin has been auctioned 29 times, of which it has sold 21 times. This indicates good demand, which may drive up the price.

4. Previous Prices: Looking at previous hammer prices, there are examples of similar coins that have sold for prices that varied significantly, from 28.88 euros to 406.71 euros, depending on the condition and other factors.

5. Recent Trends: In the most recent auctions, the hammer prices have shown a range that suggests that, although condition is a factor, there is sustained interest in this coin.

Given all this, an estimated maximum purchase price could be set in a range between 80 and 100 euros, considering that the average hammer price is 81.9 euros and that the condition of 75 may limit the price compared to coins in better condition.

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