First, you are confusing a few different types of coins.
Both of these types of coins are produced by the Austrian mint and are legal tender in Austria, but not in the rest of Europe. This means that theoretically these coins could be used to purchase items in a store in Austria for face value, and they could be exchanged for paper money at face value at an Austrian bank. However, the intrinsic value is different than face value.
The collectible coins in copper have a very low melt value. The face value of the coin, for which you can buy these coins from the Austrian Mint, is much higher than the value of the metal content. These coins have value solely because they are collectible. Being legal tender, they will not go below face value (at least in Austria), but they will only go up in value if they are sought after by collectors. Once the Mint sells out of a particular coin and is only available on the secondary market, its value is determined by the supply and demand of the collectors. However, because the metal content is worth little, don’t expect the value of these coins to rise dramatically.
Bullion coins, on the other hand, are made of precious metals that are worth much more than their face value. They are still legal tender, but their melt value and collectibility drive the actual price much higher, and you cannot actually buy these at face value.
Will I always be able to sell this copper coin for 5 euros? Who buys coins for their face value?
Because these are legal tender in Austria, yes, you should be able to get at least face value in Austria. If you can’t find a store to accept them, you should be able to exchange them at an Austrian bank. However, their collectible value will usually be higher than face value.
Is selling the coin reserved only for the country where the coin was bought?
You can sell them to anyone that wants to buy them, but their guaranteed face value is only in Austria.
Since the coin is from Austria, can it be used as the form of currency, for instance in a restaurant?
Theoretically, yes, but it is dependent on the restaurant cashier recognizing them as legal tender. Also, if the collectible value is higher than face value, it would be unwise to spend them at a store.
What's the point of buying copper coins since their melt value is far less than you are paying for it?
They may have value to collectors, but you make a good point. The metal content does not provide any significant value to these copper coins, so coin collectors are the only ones providing value to the coins.
Any tips and tricks for a guy that plans to drop 1k + euros on bullion coins?
Yes, my tip is don’t do it.
You are clearly not a coin collector yourself. Spending that kind of money on the copper collectible coins would be a very poor investment, as you don’t understand collectible coins and have no idea how to tell whether or not any of them will go up in value.
As for the true precious metal bullion coins, in addition to the fact that you don’t understand coins, you should not invest because precious metal investing is too risky for a beginning investor.