Based on the provided data, we can conduct an analysis to estimate a maximum purchase price for the coin in question.

1. Starting price: 1200 euros.
2. Average hammer price in the past: 1500 euros.
3. Estimated price at the previous auction: 2000 euros, although the hammer price was 1500 euros.
4. Condition of the coin: 100 (Uncirculated), indicating that it is in excellent condition.

Given that the coin is in good condition and has had a hammer price of 1500 euros in the past, along with an estimated price of 2000 euros, we can consider that the hammer price could be in a similar range or slightly higher.

Taking into account that the starting price is 1200 euros and that the average hammer price has been 1500 euros, an estimated maximum purchase price could be between 1500 and 1800 euros, considering that the condition is excellent and there is interest in the coin.

Therefore, an estimated maximum purchase price could be 1800 euros.