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

1. Starting Price: 350 euros. This is the initial price set for the auction.

2. Auction History: The coin has been auctioned 5 times and has sold on all occasions. This indicates a consistent interest in the coin.

3. Average Hammer Price: 321.4 euros. This is the average of the prices at which the coin has sold in the past, which can be a good indicator of the coin's real value.

4. Condition: The coin has a condition rating of 30 (BC+). This is relatively low compared to coins in better condition, which may affect its value.

5. Previous Prices: Previous hammer prices varied, but the highest was 370 euros in 2014. This suggests that, although the condition is low, there is potential to reach higher prices in the future.

Given that the starting price is 350 euros and considering the average hammer price of 321.4 euros, as well as the price history, an estimated maximum purchase price could be in a range between the starting price and the average hammer price.

Taking all these factors into account, an estimated maximum purchase price could be approximately 370 euros. This is based on the highest price reached in previous auctions and the consistent interest in the coin, although the condition is a limiting factor.