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

1. Starting price: 350 euros.
2. Average hammer price in the past: 800 euros. This indicates that, in the only previous auction, the coin sold for a significantly higher price than its starting price.
3. Condition of the coin: 85 (ebc-), which is a good condition, potentially increasing its value compared to lower-quality coins.
4. Auction history: The coin has been auctioned once and sold on that occasion, suggesting there is interest in the coin.

Given that the average hammer price was 800 euros and considering that the condition is good, an estimated maximum purchase price could be in the range of 800 to 1000 euros, depending on the demand in the current auction and the competition among bidders.

Therefore, an estimated maximum purchase price could be approximately 900 euros.