Well, I just bought a car pretty cheaply because the guy detailed it under the hood, reducing it to a code throwing undrivable machine. Cool!
If you simply clean up under the hood, you'll likely be fine.
If you clean enthusiastically with a pressure washer, you've got a good chance of pushing soap and water into connectors and such, creating some interesting electrical problems.
And if you then spray silicone all over everything to make it look pretty, you short out all kinds of things, screw up sensors, creating quite a mess for yourself.
The least step is what the previous owner did, reducing the car to the code throwing undrivable state. Towed it home, cleaned it up, replaced a few sensors, and it was 90% better. Still not perfect, but what the heck.