Maybe this is not a precise question, but could you provide a rough estimation of how much memory (both RAM and hard drive) do I need to maintain my local node in synch with the mainnet for a year? I am not going to be a block producer, but just to be in touch with the mainnet.
Probably not very much yet. There are very few transactions and actual RAM usage. Seems like actual RAM usage is ~7-8GB, and very few transactions. Will probably be able to run on entry level hardware. The blockchain should support up to 64GB RAM usage and around 500 TPS, so it's possible that your node will be subjected to that, worst case.
So you should be able to run a node on entry level hardware at this point in time, with sufficient RAM.
If EOS ever adds more RAM or CPU capacity, your node will have to upgrade.