To estimate a maximum purchase price for the coin in question, we can analyze the provided data:

1. Current starting price: 300 euros.
2. Average starting price in previous auctions: 233.33 euros.
3. Average hammer price in previous auctions: 399.29 euros.
4. Auction history: It has been auctioned 8 times, of which it has sold 7 times, indicating good demand.
5. Previous hammer prices: The hammer prices have varied, with a maximum of 420 euros and a minimum of 175 euros.

Since there is no information about the condition of the current coin, it is difficult to make a direct comparison. However, we can observe that in previous auctions, the hammer prices have been quite variable, but overall, the average hammer price (399.29 euros) suggests that buyers are willing to pay close to this value.

### Estimation of the maximum purchase price:
- Considering the average hammer price (399.29 euros) and the fact that the starting price is 300 euros, a reasonable maximum purchase price could be in the range of 400 to 450 euros, assuming the coin has an acceptable condition and that demand remains steady.

Therefore, an estimated maximum purchase price could be approximately 420 euros. This is based on the history of hammer prices and the trend of buyers in previous auctions.