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

1. Current Condition: The coin has a condition of 30 (BC+), indicating that it is not in the best shape. This will affect its value compared to coins in better condition.

2. Previous Auction Prices:
- In 2020, the coin was auctioned with a starting price of 250 euros and sold for 320 euros, with a condition of 80.
- In 2013, it sold for 280 euros with a condition of 70.
- The hammer prices have ranged from 200 to 320 euros in previous auctions.

3. Average Hammer Price: The average hammer price for this coin is 308.33 euros, which can serve as a reference.

4. Current Starting Price: The current starting price is 350 euros, which is higher than the average hammer price in the past.

Given that the current condition is significantly lower than in previous auctions, and considering that the starting price is higher than the average hammer price, it is reasonable to adjust the maximum purchase price.

### Maximum Purchase Price Estimation:
- Based on the average hammer price of 308.33 euros and the condition of 30, a maximum purchase price could be set between 200 and 250 euros, taking into account that the condition negatively affects the value.

Therefore, an estimated maximum purchase price could be approximately 250 euros.