The S2000 you'd get for the starter kit price plus engine/box and wheels would be a lot more basic than you might think. Unless it's changed from when I first looked, some of the parts in the starter kit were very basic versions of parts that almost all buyers upgrade without thinking, seats spring to mind.
That's not to say the car you'd get for that amount wouldn't be excellent dynamically, and form a very, very good building block for further development as funds allow.
The following applies to any build of course, but you need to decide on what you're aiming for with the build. Do you want to put a good "work-a-day" kit together, (reliable, no frills) or do you want to achieve something that looks like a new car.
The factory for instance, on their S2000 builds do no standard cosmetic work on the engines and boxes other to give them a basic clean; which is why you'll sometimes see the immaculate new Mega S2000's there, and when you remove the bonnet, the first thing you see is Honda's peely powdercoat on the cam cover! The drivetrain still works just as well though! (And realistically, if the factory did what some of us as home builders do in this department, the labour and parts bill could add a considerable amount).
Considerations typically overlooked in price calculations include things like giving the engine/box a spruce up and replacing consumables like the clutch. For those like me, that stripped the ancillaries of the engine to paint it and refresh the gaskets, you're looking in the region of £400 for fresh gaskets and seals to re-build, likewise most pop a new clutch assembly in there, average is probably around £300 give or take. Add £40/50 for a sump baffle plate plus welding.Then there's getting cam covers powder coated...
I bet I had little change from a grand, just prepping the engine and box for fitment, by the time you add paints, fluids etc.
On the other hand, as Sycho said, if you're prepared to "break" a whole S2000 yourself, you can save large amounts and gain an effectively free engine and box.