To estimate a maximum purchase price for the coin that will be auctioned, we can analyze the provided data:

1. Current starting price: 500 euros.
2. Average starting price in previous auctions: 780 euros.
3. Average hammer price in previous auctions: 1294 euros.
4. Auction history: The coin has been auctioned 5 times and has sold on all occasions, indicating good demand.
5. Previous hammer prices: The hammer prices have varied, but the highest was 2600 euros in a recent auction with a grade of 95.

Since there is no information about the condition of the current coin, it is difficult to make an accurate estimate. However, considering the average hammer price of 1294 euros and the fact that the coin has a history of successful sales, we can make an estimation.

If we take into account the average hammer price and the fact that the starting price is relatively low (500 euros), we could estimate that the maximum purchase price could be in a range between the average hammer price and the highest recorded price.

Estimation of the maximum purchase price:
We could consider a maximum purchase price of around 1500 to 1800 euros, taking into account the price history and the lack of information about the condition. This would allow for a competitive bid without exceeding the historical hammer prices.