To estimate a maximum purchase price for the coin in question, let's consider the following factors:

1. Starting price: 1200 euros.
2. Average hammer price in the past: 1700 euros.
3. Condition of the coin: 100 (S/C), indicating it is in excellent condition.
4. Estimated price at the previous auction: 2000 euros, although it was not reached at the auction.

Given that the coin has a high condition rating and has had a hammer price of 1700 euros in the past, it is reasonable to consider that the hammer price could be in a similar range or slightly higher, especially if there is interest in the current auction.

Taking into account that the starting price is 1200 euros and that the average hammer price has been 1700 euros, a maximum purchase price could be around 1800-2000 euros, as the interest in the coin and its condition could attract more bidders.

Therefore, an estimated maximum purchase price could be 2000 euros.