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, but not among the best qualities (FDC).

2. Starting Price: The starting price is 350 euros, which is a good starting point.

3. Sales History:
- The average hammer price in previous auctions is 298.34 euros.
- In past auctions, hammer prices for coins with similar conditions (around 70-80) have varied, but some have reached hammer prices of up to 387 euros (as in the March 2020 auction with a condition of 95).

4. Price Trends:
- In recent auctions, hammer prices have been in a range of 300 to 400 euros for similar conditions.
- The trend seems to be that prices are slightly increasing, especially for coins in good condition.

5. Estimation Based on Historical Data:
- Given that the condition of this coin is 75, and considering that the highest hammer price in the past for a similar condition was 387 euros, we can estimate that a reasonable maximum purchase price could be in the range of 400 to 450 euros.

Conclusion: Based on all these factors, an estimated maximum purchase price for this coin could be approximately 400 euros. This allows for a margin for competition in the auction and reflects the current market value for coins of similar condition.