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 of 75 (MBC), indicating that it is in an acceptable state, but not of the highest quality.

2. Historical Prices:
- The average hammer price for coins of similar condition (75) has varied, but there are examples of auctions reaching up to 650 euros (as in the 2018 auction).
- In previous auctions, hammer prices for coins in condition 75 have ranged between 90 and 650 euros.

3. Starting Price: The starting price is 50 euros, which is relatively low compared to previous hammer prices.

4. Sales Trends: The coin has been auctioned 109 times, with a 71% sell-through rate (78 times sold). This suggests there is consistent interest in this coin.

5. Average Prices: The average hammer price for all auctions is 552.49 euros, indicating that, in general, coins of this reference have good market value.

### Estimation of Maximum Purchase Price
Given that the coin has a condition of 75 and considering previous hammer prices, a reasonable maximum purchase price could be in the range of 300 to 400 euros. This is based on the fact that, although the condition is not the highest, there is a price history suggesting it can reach higher figures at auctions.

Therefore, an estimated maximum purchase price could be 350 euros. This price reflects a balance between the current condition and the historical interest in the coin.