Looking to setup a lightning node, but what are the EXACT requirements? Do you have to install bitcoin core and fully have the blockchain downloaded of 150gb+?
A lightning node needs a way to refer a copy of the blockchain that it trusts, so the node can either have it locally or might refer to one using neutrino. That would allow you to run it without an external hard drive at all. You could run a lightning node on a rpi off just an ssd card by this method.
Yes, you need to be able to query every transaction within the whole blockchain. Two ways to achieve that:
Run your own full node (with transaction indexing [txindex=1] and no pruning), that requires about 200 GB to store everything (see my Raspberry Pi guide for additional information)
Trust a remote Bitcoin node to tell you the truth, eg. using Neutrino. This will be the enabler for all mobile applications and is currently tested on Bitcoin testnet. No mainnet release yet.