What is the memory usage for checking transaction inputs?
You are right UTXO set is stored on disk on .bitcoin/chainstate directory using leveldb. It is stored in compressed state and the current size is around 1.5GB

To speed up access bitcoind uses an in memory cache which can be configured using -dbcache option.  


  So, I assume the chainstate-db is stored on disk, but accessed
  multiple times per second to check transaction inputs. What amount of
  memory usage does this induce on a node?


Accessing the leveldb database itself is like accessing any other nosql database. You basically lookup the key which is the utxo and get a value as output which you need to deserialize and store in a memory variable