-500 € is a good price for this type of channel?
No one can advise you on the valuation as the basic metrics like views per video and number of subscribers aren’t very useful. Not all subs, views, niches, etc. are worth the same. Assuming the channel is pretty much completely dead, niche isn’t anything special, etc. then the price is pretty average. Just know that you’re mostly paying for inactive subscribers, for social proof. I’m guessing the asset is making almost no money, gaining no new subscribers, will have almost 0 active subscribers and no serious fans. For some, there is value in this kind of channel but for most people, probably not worth buying. Don’t take my word for it, do your own research; I can’t really evaluate a channel with this kind of info and don’t have the time to anyways.
-If I will buy the channel, will changing its name cause a shadowban from Youtube?
It’s possible but probably not.
-Are the videos in the channel recommended to be deleted or made private? (I’m a whole lot)
I usually suggest not deleting videos as this sometimes causes temporary issues with getting new videos ranked and suggested. To be safe, upload your first video with all the others still public, over time you can unlist the videos slowly to ensure no problems are created. Ultimately you can private or delete them all but get other videos, views and watch time up there to replace them first.
-How long it’s gonna take for me to resurrect the channel? Do I have to post consistently?
You might not be able to resurrect the channel. If the subscribers are inactive, assume they’ll always be inactive. Maybe you’ll get 10’s, hundreds or thousands of views on new uploads (just depends on the kind of fans the channel currently has). But if you change the content to something different, those active viewers may die off to almost nothing. You’ve gotta make a guess as to how active the channel is, then you can work out how long it’ll take to increase the growth rate. You may just be buyig a dead channel and building from the ground up just like you would a brand new channel.
-Log in the account from different IP, will raise a flag like on Instagram?
The current recommendation for YouTube channel acquisitions is to take ownership of the channel via management settings, where they can add your own email account as the primary owner and revoke their own access. If you take their email address, they can report it as hacked at a later date to steal it back. It’s much harder to get scammed if you do an ownership transfer instead, with the seller adding you to primary owner and revoking their own access themselves.