If possible, Kassel will be the nearest and good choice. From here you need 30 Minutes with public transport - but we havn't jet checked this. In some months we are going
to have more reliable information.
Hannover is a good choice, it's big enough and you should get a flight
However if you need a cheap flight and you would get a rental car anyway, you might wanna
consider Düsseldorf Weeze, too. This is not the closest Airport though, but it might be the
cheapest flight option for you.