Official Varnish repositorie (https://packagecloud.io/varnishcache) doesn't offer any release for Buster yet. 6.3 was added since 2 days, but not for Buster yet.
Handshake is currently in testnet and the auctions last a few days, but on mainnet the auctions last ~2 weeks. Names are open for bidding for the first 5 days, and then the next 10 days are for bids to be revealed.
There isn't a client side API. It's an HTTP GET for read, which returns an application/json response, and a POST of the same to create, other http ops for delete (DELETE), update (PUT).
I assume you would use https.request() in node or some abstraction that sits on top of that.