To estimate a maximum purchase price for the coin in question, we can 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.

2. Auction History:
- The average hammer price in previous auctions is 362.11 euros.
- The coin has been auctioned 21 times, of which it has sold 20 times, indicating good demand.
- Previous hammer prices for coins with similar conditions (around 75) have varied, but there are examples of auctions that have reached up to 700 euros for coins in better condition.

3. Starting Price and Estimates:
- The current starting price is 350 euros, which is slightly higher than the average starting price of 287.38 euros.
- The estimated price of 750 euros in a previous auction (for a coin with a condition of 95) suggests that there is potential for high prices, but this does not directly apply to the current condition.

4. Price Trends:
- In recent auctions, hammer prices have ranged from 280 to 720 euros, depending on the condition.

Based on these factors, an estimated maximum purchase price for the coin with a condition of 75 could be in the range of 400 to 500 euros. This is based on the auction price history and the current condition, considering that the starting price is 350 euros and that there is good demand for this coin.

Therefore, a reasonable maximum purchase price could be 450 euros.