To estimate a maximum purchase price for the coin in question, we can analyze the provided data:

1. Current starting price: 200 euros.
2. Coin condition: 70 (MBC-), which indicates good condition, although not the best.
3. Auction history: The coin has been auctioned 4 times in the past, and all have successfully sold. The average hammer price in previous auctions is 212.5 euros.
4. Previous hammer prices: The hammer prices in previous auctions with the same condition (70) have been 280, 180, and 170 euros.

Given that the average hammer price is 212.5 euros and considering that the current starting price is 200 euros, it seems that the market is willing to pay a little more than the starting price. However, the highest hammer price in the past was 280 euros, indicating potential for a higher price.

Taking all these factors into account, an estimated maximum purchase price could be set in a range between the average hammer price (212.5 euros) and the highest previous hammer price (280 euros).

Therefore, a reasonable estimated maximum purchase price could be 250 euros. This reflects a balance between the price history and the current condition of the coin.