So many people buy a car planning to build it up, but then the idea goes stale because it gets expensive and complicated. Often, the problem is starting off with a car that's already expensive and complicated. It's a truism that an expensive car bought ten years ago is going to be an expensive car to maintain now, and if all the money is going into maintenance then little is going to go into modification.

That's not a hard and fast rule though. Older cars tend to be simpler because the available technology was simpler and/or legislation hadn't overcomplicated anything. A great example of that is the pre-2000s E36 generation BMW 3-Series and the new millennium's E46 generation. While you'll still immediately want to refresh the cooling system on an E36, there's going to be less electrical issues due to less complexity, parts are freely and cheaply available, and bushings are simple and easy to replace. That means more time and money can go into bringing the suspension and handling up to date.