What is the memory usage for checking transaction inputs? Whenever a transaction is received by a node, the node verifies its validity. To that end, it checks for each transaction input whether that input is part of the UTXO set. It is my understanding that the UTXO set is stored in the chainstate database, which is held in a LevelDB structure.

I previously thought that he UTXO set was kept available in memory, but since I've learned that it is currently about 1.2GiB in size that seems unlikely.

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?
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