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 a medium-good quality. This is relevant because coins in better condition tend to reach higher prices.

2. Previous Prices: Looking at previous auction prices for coins with similar conditions:
- In 2020, a coin with a condition of 70 was auctioned for 420 euros.
- In 2018, a coin with a condition of 75 was auctioned for 400 euros.
- In 2022, a coin with a condition of 75 was auctioned for 440 euros.
- In 2023, a coin with a condition of 75 was auctioned for 600 euros.

3. Average Auction Price: The average auction price for previous auctions is 501.25 euros, which can serve as a good indicator of market value.

4. Starting Price and Average Starting Price: The current starting price is 350 euros, and the average starting price in previous auctions is 397.22 euros.

5. Sales Trends: The coin has been auctioned 11 times, of which it has sold 8 times, indicating good demand.

Based on these factors, especially considering the most recent high auction price (600 euros) and the average auction price (501.25 euros), an estimated maximum purchase price for this coin could be around 550-600 euros, given that the condition is good and there is demonstrated interest in previous auctions.

Therefore, an estimated maximum purchase price could be 550 euros.