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

1. Starting price: 350 euros.
2. Average hammer price: 420 euros. This indicates that, in the past, the coin has had good interest and has reached a price higher than the starting price.
3. Condition: The coin has a condition rating of 70 (VF-), which suggests that it is in fairly good condition, although not the best possible (UNC).
4. Auction history: It has been auctioned once and sold on that occasion, indicating that there is demand for this coin.

Given that the average hammer price is 420 euros and considering that the condition is good, an estimated maximum purchase price could be in a range between the average hammer price and an additional 10-20%, as demand may vary.

Therefore, an estimated maximum purchase price could be:

Maximum purchase estimate: between 450 and 500 euros.

This is based on the good condition and the interest shown in the previous auction.