Is it possible, when adding a new ad, to make some kind of hints for the user, based on keywords, in which section is it better to add an ad?

Not easily.
You would need to have keywords list for each category (rich keywords list) and then smart function to propose better category

It would be great to buy such a plugin

It is quite lot of work for such mi or functionality 

How much will this "pretty big job" cost?

@atba
Maybe 100-200, all depends on final functionality that should be included in such plugin/mod.

The functionality is already described above... The user enters a title and is offered a suitable section based on keywords. From the drop-down list, a person selects the right one, if there is a list, and proceeds to fill in the description and everything else. This is done everywhere on all known more or less Internet resources. P.S. I will be ready to pay as much as you voice, but on condition that it will be an individual revision (although you are unlikely to agree to this), because paying 5-10 times more so that all other users have the opportunity to buy a plugin for 30 euros is not serious. And it will definitely be in demand, because it will greatly simplify the work of moderators when checking ads placed in the wrong category. Why the wrong one? You yourself understand that with a huge amount of information (and resources of such a plan imply this), the user will often not waste time, but will place an ad in the first section that comes to hand, which, as it seems to him, fits the description of his product. Something like that). In general, if there are those who want to buy this revision, let's ask the developer together to create it. Perhaps we will collect even more money together at the start?