To estimate a maximum purchase price for the coin that will be auctioned, we will consider several factors, including the coin's condition, previous auction prices, and market context.

1. Coin Condition: The coin in question has a condition of 70 (MS-). This indicates that it is in an acceptable state, but it is not of high quality (PRF).

2. Previous Auction Prices: By observing the auction prices of coins with similar conditions (70 and 75), we can see that the auction prices have varied. For example, in recent auctions, coins with a condition of 70 have reached auction prices ranging from 30 to 100 euros, depending on demand and other factors.

3. Average Auction Price: The average auction price for this particular coin is 99.9 euros, suggesting that, in general, buyers are willing to pay close to this price.

4. Starting Price: The starting price is 18 euros, indicating that there is considerable room for the auction price to rise.

5. Market Trends: Given that there are 18 similar coins that will be auctioned in the coming days, this could influence competition and, therefore, the final price.

### Estimation of Maximum Purchase Price:
Taking all these factors into account, a reasonable maximum purchase price for this coin could be in the range of 100 to 120 euros. This is based on the average auction price and the coin's condition, as well as the price trends of similar coins in the market.

It is important to remember that the final price may vary depending on the competition in the auction and the interest of collectors.