Turbocharging your car's stock engine will substantially increase the engine's horsepower potential. However, there are many upgrades you can perform on your stock engine that can be utilized when installing a turbo system in the future, allowing for maximum turbocharged horsepower potential.
While it is possible to install a turbo without using a kit, it is not recommended unless the installer has done it before and has access to a full machine shop. Can you drive with a blown turbo? The longer you drive your car with a blown turbo, the more damage the engine will have and therefore the more costly it will be to repair. The longer the blown turbo is left without repair, the more damage can be caused to the car's engine.
Is it cheaper to build your own turbo kit? Yes it cheaper but it's way more stressful. Building of the kits is easy part. It's the trial and error of clutching and getting the after-martkets to adjust their box to fit your turbo that takes the time and money.