i3 vs i5 (base model). You can upgrade either to the i7 for $300.
Both start with 8 GB memory, you can upgrade to 16 GB for $200, 32 GB for $600, or 64 GB for $1000.
If you get the i3, you can upgrade to 512 GB of storage for $200 (essentially, that and the i3 -> i5 upgrade for $100 is a processor + storage upgrade disguised as a different model). You could also upgrade to 1 TB for $400 or 2 TB for $800.
A 10GB Ethernet NIC is the last option, available for $100; I guess they realize these are likely to be used in farms where network transfer will be important.