Shimano 105 5700 in 2019 I want to begin with road biking as a sport and are currently trying to find a decent bike.

Some friends recommended to get the Radon R1 2019 with a complete Shimano 105 R7000 groupset.

In a bike-store I found a new no brand bike with a complete Shimano 105 5700 groupset.
It also features an aluminum frame, a carbon fork and weighs about 10.5kg for less than 300€.
Is it still relevant in 2019, as it is almost 10 years old, or is it comparable with newer Shimano Sora groupsets via trickle-down?

There seems to be a lot of myths and opinions on various groupsets, but is there some "evidence", which indicates a different performance?
This is very much opinion based question. I'd say that 5700 is still above current day Sora and a very solid choice indeed. I'll also argue with some of the comments that 10.5kg is heavy, especially at the price point of 300 EUR. As a comparison Triban 520 RC (praised to be extremely good value alu/carbon bike for packing partial Shimano 105 groupset in 750 EUR) is 10.3kg. There are very few bikes under 9kg that cost less than 1000 EUR and probably none under 10kg for 500 EUR or less.

Actually, at this price point, this is probably way above anything else you might be able to get.