Hello rayigen85,
Unfortunately, a valuation can hardly be carried out based on this. The description "Umkreis 30km Hannover" alone contains fluctuations of easily €200/m² in the standard land value price, which corresponds to a range/variance of €70,000 for your value of 350m². The first reference point is, of course, the seller's price, and you should simply look at 5-10 properties in the area to get a feel for the prices there. If you do not want to undertake this effort, a valuation report can be helpful. However, it does not have to be, and it can also become expensive if you commission 5 reports for 5 properties and then do not buy any of them.
Best regards
Dirk Grafe