If you're in America I can ship you a specific model of ThinkPad for $120-180 plus shipping depending on options. Generally Core i5 processor, up to 32GB RAM, up to two internal SSDs for storage redundancy highly recommended for lightning nodes. Power consumption is as low 5W idle average 10W.
You can get a steal on used hardware - for example sub $200 older mac mini, or older business machine.
Check out nostr:npub1pfezegswrn8lpt0aerp2hvyhj4lsuzln9kcccs5p7qch2m2sawxsxcelvf and nostr:npub1txwy7guqkrq6ngvtwft7zp70nekcknudagrvrryy2wxnz8ljk2xqz0yt4x guides on this. Sometimes they sell machines directly.
Skip the raspi. Its underpowered.
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nostr:npub1pfezegswrn8lpt0aerp2hvyhj4lsuzln9kcccs5p7qch2m2sawxsxcelvf which mobile client do you use?