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

1. Starting price: 1200 euros.
2. Auction history: The coin has been auctioned 20 times, of which it has sold 17 times. This indicates good demand.
3. Average hammer price: 1136.93 euros, which is a good indicator of the price buyers are willing to pay.
4. Condition: There is no information on the current condition of the coin, which may affect the price. However, in previous auctions, hammer prices have varied depending on the condition, with condition values ranging from 70 to 95.
5. Previous prices: Hammer prices have ranged from 580 to 3600 euros, depending on condition and other factors.

Given that the starting price is 1200 euros and the average hammer price is 1136.93 euros, an estimated maximum purchase price could be set in a range that considers demand and price history.

Taking all these factors into account, an estimated maximum purchase price could be approximately 1500 euros. This price reflects a margin over the starting price, considering the possibility that the coin has a favorable condition value and the trend of prices in previous auctions.