You mean a guard node ? What would be the spec for such a node, i suspect this should be a VPS somewhere in a data center, right?
Discussion
https://community.torproject.org/relay/setup/guard/
I don't think it has to be substantial, the Tor configuration can be tuned to fit the 'hardware'.
I was running one on our VDSL without an issue, when we upgraded to fibre I just increased the bandwidth limits a little.