To estimate a maximum purchase price for the coin in question, we can analyze the provided data, including the starting price, current condition, and previous auction hammer prices.

1. Starting price: 300 euros.
2. Current condition: 75 (MBC).
3. Average hammer price: 510.79 euros.
4. Hammer price history:
- The coin has had hammer prices that vary, but generally, the hammer prices have been higher than the starting price.
- In previous auctions, hammer prices have reached up to 2200 euros in a recent auction with a condition of 85.

5. Price trends:
- The overall trend shows that coins with similar condition (75) have had hammer prices ranging between 280 and 600 euros in previous auctions.
- The most recent auction with a condition of 70 had a hammer price of 600 euros, suggesting that the price could be in a similar or slightly higher range for the condition of 75.

6. Estimation:
- Given that the average hammer price is 510.79 euros and considering that a condition of 75 is quite good, an estimated maximum purchase price could be in the range of 600 to 700 euros, taking into account that the highest hammer price in the past was 2200 euros, but that was for a higher condition.

Therefore, an estimated maximum purchase price for this coin could be approximately 650 euros.