To estimate a maximum purchase price for the coin in question, we can analyze the provided data, including the starting and hammer prices from previous auctions, as well as the condition of the coins.

1. Current starting price: 700 euros.
2. Average hammer price in the past: 261.67 euros.
3. Previous hammer prices:
- The highest auction was 900 euros (condition 50).
- The lowest auction was 25 euros (condition 25).
- There are several auctions with hammer prices between 50 and 550 euros, with varying conditions.

4. Condition: There is no information about the condition of this coin, which makes the estimation more uncertain. However, observing the conditions of previously auctioned coins, it can be noted that coins in better condition tend to reach higher prices.

5. Trends:
- The general trend shows that hammer prices can be significantly lower than the current starting price, especially if the condition is unknown.
- The average hammer price of 261.67 euros suggests that the starting price of 700 euros is quite high compared to what has been sold previously.

### Estimation of the maximum purchase price:
Given that the starting price is 700 euros and considering that the average hammer price is 261.67 euros, a reasonable maximum purchase price could be in the range of 300 to 400 euros, assuming that the buyer is willing to pay a little more than the average due to the uncertainty in the condition.

Therefore, an estimated maximum purchase price could be approximately 400 euros. This allows the buyer to take into account the possibility that the coin may have a higher than expected condition value, but also reflects the reality of previous hammer prices.