Even the early versions of OS X are not updates to OS 9. Instead, all versions of OS X install alongside OS 9 - both can be present on the same volume without needinng to partition the drive, and both be usable on machines which support both.
There are no initial versions of OS X (any version) which can be downloaded. If you want to have Cheetah (10.0.x) installed, you would need to somehow obtain a retail install disk for it. You could then download updates for later versions of Cheetah.
However, if you want to end up at Jaguar, OS X 10.2.x, you should not bother with any earlier versions of OS X. Get a retail copy of Jaguar on CD and install it directly.
It would help us help you better if we knew the exact model (the Apple designation) iMac that you have, its processor speed, and how much RAM it has. There are some minimum requirements for various OS X versions, plus experience with what versions run best on what hardware.
This Apple KBase article can help you identify which iMac model you have -
You need to check if your iMac requires a firmware (Boot ROM version in System Profiler) update. Mac OS X: Available firmware updates http://docs.info.apple.com/article.html?artnum=86117 If an update is required, the update must be installed (using OS 9) BEFORE you install OS X.
Here's the spec pages for the various iMac models, http://www.everymac.com/systems/apple/imac/index-imac.html , however, I don't see a spring 2000 model. Depending on which 2000 model you have, you can install Panther 10.3 or possibly Tiger OS 10.4. But you will need a minimum of 256MB RAM. Some of those models came with only 64MB.
For information on your Mac (model, date of production, etc) enter your serial number at this site. Information, based on the serial number, will be shown. http://www.chipmunk.nl/klantenservice/applemodel.html or last 3 characters of S/N at this site http://www.everymac.com/ultimate-mac-lookup/ Post back what model.
I have OS 10.2 .ISO files... will that work or no?
I have no idea how to install a new OS using an .iso file. I've always used a retail install CD/DVD.
The iMac (Slot Loading) model was released in Oct 1999, and discontinued in Jul 2000.
The iMac DV (Summer 2000) model was released in Jul 2000, and discontinued in Feb 2001.
In either case, if the original machine (CPU) speed is 400MHz, it can use OS X 10.4.x; if it is less than 400MHz, it is limited to OS X 10.3.x.
The machine will also need to meet the other hardware requirements for the OS X version desired - minimum RAM, sufficient space on the hard drive, etc.