To estimate a maximum purchase price for the coin in question, we can analyze the provided data and consider several factors:

1. Current Condition: The coin has a condition of 75 (MBC), which indicates that it is in an acceptable state, but not of the highest quality.

2. Auction History:
- The coin has been auctioned 12 times, of which it has sold 9 times. This indicates good demand.
- Previous hammer prices varied, but there are examples of hammer prices reaching up to 850 euros in superior conditions.

3. Starting and Hammer Prices:
- The average starting price is 433.33 euros, and the average hammer price is 497.78 euros.
- In recent auctions, the hammer price has been 500 euros for a coin in condition 90 and 380 euros for one in condition 75.

4. Price Trends:
- Hammer prices for coins with similar conditions (75) have been around 380 to 500 euros in recent auctions.

Based on this data, an estimated maximum purchase price for the coin in condition 75 could be in the range of 400 to 500 euros, considering that the starting price is 350 euros and that the hammer price history suggests it could reach up to 500 euros in a competitive auction.

Therefore, a reasonable estimated maximum purchase price would be approximately 500 euros.