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

1. Starting price: 350 euros.
2. Current condition: 70 (MBC-), which is an average condition.
3. Auction history:
- In 2020, a coin with a condition of 85 sold for 380 euros.
- In 2022, a coin with a condition of 95 sold for 420 euros.
- In 2023, another coin with a condition of 85 sold for 335 euros.

4. Average hammer price: 391.25 euros, indicating that, in general, coins of this reference tend to sell for above their starting price.

Given that the current condition of the coin is 70, which is lower than the conditions of 85 and 95 of the previously auctioned coins, it is reasonable to expect that the hammer price will be lower than the 380 and 420 euros of the better-preserved coins.

However, the starting price of 350 euros suggests that the seller has expectations of a relatively high hammer price. Considering the price history and the current condition, an estimated maximum purchase price could be in a range between 300 and 350 euros, as condition is an important factor in the value of the coin.

Therefore, an estimated maximum purchase price could be approximately 350 euros, but it might be more prudent to consider a range of 300 to 340 euros for a more conservative offer.