Hi @eucador
The T400 supports a maximum of 8GB of DDR3L-1600 PC3-12800 sodimm ram i.e. 2 x 4GB
Depending on how much you wish to spend the laptop supports 2.5" sata SSDs ''up to'' 4TB capacity.
With this amount of ram and storage you can install and run Win10 bearing in mind that support for Win 10 ends on Oct 14 2025.
You can also install Linux as well for longer OS support. (depending on which Linux version you choose)
Here's the [https://thinkpads.com/support/hmm/hmm_pdf/43y6629_05.pdf|hardware maintenance manual] that may help.