I'm not sure about anything, but the client, as I've been usenetting without thinking much about stuff for almost a decade now.
- eweka.nl (share your access with friends and family - I've shared mine with like 8 people some time in the past)
- nzbgeek (lifetime API access was like 30€ - well worth it), I also know about nzbplanet, but I don't have a premium account there. You can try these out without paying to see if their offer suits what you're looking for, but you won't get API access of course and you'll be limited in the downloads.
Last, but not least: sabnzbd is the gold standard for clients. Runs on everything: from a raspberry pi to your Desktop PC.