Your case is good for air cooling.
You have a 200mm front intake and 171mm height available.
Not a greek reader, but I think you can get a I5-14600K for 300e.
You should be able to find a lga1700 DDR4 motherboard for about 200e.
Nobody overclocks any more; a Z690/Z790 motherboard will be fine.
You may need such to handle 4 sticks of ram.
But, B660 and B760 MATX motherboards are less expensive and some do have 4 ram slots.
You can reuse your DDR4 ram. Intel is not sensitive to ram compatibility.
You would need a lga1700 adapter for your cooler.
But a cooler like the thermalright peerless asassin is a twin tower cooler that might go for 40e.
Your I7-7700K has 8 processing threads and a passmark rating of 9654. That is when all 8 threads are fully utilized.
Not likely to happen unless you play multiplayer games.
The single thread performance rating is2725.
The I5-14600K has 20 threads and a rating of 40395 with a single thread rating of 4294.
Do not try to save a few euros with the 14600KF and not get integrated graphics.
Such pennypinching always seems to bite you when you have GPU issues.
A ryzen build will be competitive on the cpu/mobo price/performance.
But your ram is not likely to be compatible.
Ryzen is different and since you now use intel, you will have fewer issues.