To estimate a maximum purchase price for the coin in question, we can analyze the provided data, including the starting price, the average hammer price, and the condition of the coin.

1. Starting price: 350 euros.
2. Condition of the coin: 75 (MBC), indicating a medium-good condition.
3. Average hammer price: 417.93 euros, which is a good indicator of the coin's real value in previous auctions.
4. Price history: We observe that in previous auctions, hammer prices have varied depending on the condition. For example, in an auction with a condition of 90, the hammer price was 400 euros, and in another with a condition of 95, the hammer price was 1300 euros.

Since the condition of the coin is 75, which is lower than the conditions of 90 and 95 that have reached higher prices, we can infer that the hammer price will not reach the highest levels.

However, considering the average hammer price of 417.93 euros and the starting price of 350 euros, an estimated maximum purchase price could be set in a range that reflects the condition and price history.

Estimated maximum purchase price:
We could consider a maximum purchase price of around 450 euros. This is based on the average hammer price and the fact that the condition is good, but not excellent. This price also allows for a margin over the starting price, which could be attractive to a buyer at the auction.