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

1. Starting price: 1800 euros.
2. Average hammer price in the past: 4600 euros.
3. Condition of the coin: 70 (MBC-), indicating good preservation, although not the best possible.
4. Auction history: It has been auctioned once and sold on that occasion, suggesting there is interest in the coin.

Since the average hammer price is significantly higher than the starting price, and considering that the condition is good, we could estimate that the hammer price might be in a range close to the previous average hammer price.

However, given that it has only been auctioned once and the market can vary, it is prudent not to set the maximum purchase price exactly at the average hammer price.

Taking all these factors into account, an estimated maximum purchase price could be between 4000 and 4600 euros, as this reflects the interest in the coin and its good condition, but also considers the possibility that the current market may not reach the previous average price.

Therefore, an estimated maximum purchase price could be 4500 euros.