i test at the moment rnostr, supports a lot of nip’s and the developer is very responsive and fast in implement of new feature.

https://github.com/rnostr/rnostr

also a very good choice is strfry, has a lot great featurs, if you like to sync with other relays or stream your post to other relays.

https://github.com/hoytech/strfry

about the specs, this depends on your needs. if you just run a relay for you then you don’t need a very beefy machine. i run my relay on a vps with 1 cpu and 1 gb ram. and this run’s very smooth.