Hi Rookie,
I haven't road raced since 97, but I might be able to help a little.
First advice - do it! I spent an absolute fortune over the ten years I raced, and never set the world on fire - but I loved it and there are memories there that time will never erase. We are in the enviable position of having one of the worlds best GP tracks in our own back yard, so if you have the desire then jump on in and give it a go.
Modern or historic?
Generally speaking, the former is faster, more expensive and more serious than the latter. You owe it to yourself to try a modern racebike at least once in your life....
Clubs?
I used to be a member of Hartwell MCC, which had a very strong club championship in the day. I hear that has dropped off a little, and that the Interclub series is the way to go these days. For the Interclub series, Preston and Sandringham clubs were well regarded. I think any of the good roadracing clubs will offer you a good range of classes from buckets upwards.
Bikes
Buckets are cheap, but I never saw the value in signing up for an opportunity to ride beyond the open road speed limit - and then choosing a bike that could barely do it. RGV's and NSR's are a heap of fun, and pretty simple to work on, although you will need to search hard for one that isn't flogged out (watch out for exhaust powervalves on the RGVs). If your budget can stretch to $5000-$6000 and you are mechanically handy, 125GP bikes can be a hoot, and not too expensive to run - but that is probably my passion overwhelming my common sense here...
On the historic front, the mid-80s Yamaha RZ's are a cheap entry point that will take you as fast as you would want to go, at a budget friendly cost. Lots of setup knowledge around for them too. I'd probably do that.
Bike preparation
I assume you would do your own? Needless to say, it needs to be meticulous, as the consequences of fluid leaks or a bike failure at 200kmh are a bit more serious than VMX speeds. A good club will help introduce you to the finer points of setup, or there are some people on these boards that might be able to help you too, (myself included if your standards are appropriately relaxed...)
Riding gear
This is something you must not skimp on. I wouldn't consider a return to` the track without budgeting at least $2000 to good riding gear. There is the safety aspect of course, but there is also the functionality / comfort factor (e.g. helmets with visors that don't seal, leathers that don't fit, clumsy boots that have a poor feel for the pegs)
Riding tuition
There is a guy on these boards who could teach you anything you wanted to know about road racing - I'm sure he will jump onboard this thread...
Final Word
Go for it, live the dream. I wouldn't have traded my road racing days for anything.
Let me know if I can help
Geoff