I wanted to create a tool which finds nearby dropped items.
My idea: Activate tool (like the tracker) and do something like play a sound or display a text when there are dropped flints or whatever nearby.
Is it possible to do something like that and if yes, how?
Sounds possible but you will need to learn how to blueprint script to achieve it. It would be possible to upon activation look at all instances of a class, calculate how far they are from the player, and then do whatever event if it meets your criteria.
I definitely need to learn more about the graph scripting. But wouldn’t that be a little heavy on the game to get every item on the map of a certain class. Depending on the class, there could be a lot of them (i.e. ****).
Certainly this may be resource intensive, but you may find UE to be more resilient than expected provided there isn’t something particularly inefficient about the code. I recommend (just like they do with the new transponders) to limit how often the device can be activated. I would say look at how the transponders are coded but that is handled by source code and not blueprint graph, but doesn’t mean it cant be done. The hardest thing to likely tackle is restricting the search to only objects within a certain radius rather than fetching everything of a class and determining how far away it is.
I will likely be going through a similar possibly huge performance impacting change to my mod by allowing cpu collision based particles.
First I have to find out how to get the stuff I need in the graph editor but luckily UE4 is a big engine with lots of tutorials 
And yes I wanted to limit it to only activate once every few seconds. Once it’s done, I can check how big its impact on the performance really is.
It all depends on how efficient and persay “proper” the blueprinting is. If you are having it check every tick, then yeah its going to nuke performance. If you otherwise were to do it via a tick every 5-10 seconds, or another safer method, then you could control the impact. There is lots of ways around this 
If you haven’t already, I suggest checking out this thread I’m compiling of very useful info/tutorials/etc. A vast chunk of the tutorials cover blueprinting so you should find alot of it VERY useful for this!