Good luck with rate limit issues, I've been there.

Proxies to the rescue. And take your time - whois data don't change that often and you can know in advance when it might (E.g. expiry date).

Also been there with Nominet. Ended up implementing something with their special Domain Availability Checker only to find that it couldn't handle a single domain spanning two ip packets/tcp segments (wtf?!). After asking them about it a few time the official answer turned out to be: don't do that then.