I could go on for hours about racing zeds but Ill be brief (sort of)
In the Vic state series you car run in Marquesports or sports sedan classes. with a $15,000 car (thats actually close to what I paid for my 240z race car) you would have been competative in marquesports about 10yrs ago . Now its all very highly modified cars. You need to be doing under 1:41 at the island and around 1:14 at Sandown to be in with a chance and a zed to marquesports regs wont do that. On the sports sedan side I havent looked into it to much but the rules changed this year and not zeds can run but it would cost heaps to get a zed up to those specs.
After yrs of racing in marquesports I decieded I would put a bigger motor in my 240z and enter as an invited car under 2A sportscar rules. even with the 2A rule freedom I cannot afford to get down to the lap times the really quick Marqusportscars are doing. So If basically if you want to run in state series you wont win in a zed (at least not in Victroria ) so you have got to be doing it for fun (which is hopfully the path I am going down) and cause you love zeds.
On the go kart front I wouldn't bother. I had a stint in karts a few yrs ago and its got really pricy for your licence (more than circuit racing cams) and track fees and the racing is really rough.
The other option could be to build or buy a historic zed race car which can be fairly competative.
In 2002 I ran a whole season (6 rounds) and spent around $9,000.00
this included and engine refresh, second hand slicks, entry, accomidation etc etc this was doing all the work on the car incluing engine rebuild myself as it would probably cost another few grand if I paid someone else to do it.
Ash