Damn, how much should i consider spending on a fast reliable car?
A lot. As far as I'm aware it's very, very rare for anyone to have a fast, reliable car first try. Modifying cars isn't something you can just jump into by throwing money at them, unless you have a very good mechanic(s) who is willing to spend a great deal of [billable] time working on your car for you. Even then, getting the right work done takes knowledge, restraint, and careful planning. You can't just say that you want a reliable, fast modified car and have it appear out of thin air. In my opinion you're better off starting with something reliable, cheap, and fun, or buying something that's already reliable and fast and making small changes to it. Most of the people who own reliable, fast cars have years or even decades of experience with project cars and know exactly what they need to do. More importantly, they know what they want. I could tell you to go out and buy a WRX and stick a massive turbo on it, or I could tell you to buy an MX-5 and drop an LS in it (neither of which is a recipe for reliability without the proper expertise and a lot of money). They're totally different approaches, and so it's important to actually try things out to find out what sort of things you really want from a car. Driven wheels? Gearbox? Engine placement? Seats? Interior space? Comfort? Steering feel? Handling balance? Power? Grip? Torque? There's no single answer, so if you're absolutely set on modifying a car then I recommend starting with something cheap and going from there.
Something else I'd like to point out is the range of technical problems you'll likely face. Unless you're starting with a car that already makes close to your target power output, you'll need to take a careful look at every component in the drivetrain to see if it can handle the power. You'll have to make sure the gearbox can handle the power, and you may need to upgrade the clutch. You may also need to upgrade the diff, which is a good opportunity to get a decent LSD. With more power you'll also need better brakes, and that could potentially necessitate larger wheels. In addition, you'll have the potential to be entering and exiting corners much faster and you'll be putting down more power through them, so you'll want to take a look at the suspension to see if it needs altering to handling the additional performance. The engine itself is another area which will need upgrades. Depending on how upgraded it is you may need to upgrade peripherals like the cooling system and lubrication system. You'll likely want to upgrade the exhaust and potentially the intake, as well as the fuel system. And of course, you'll almost certainly need to retune or replace the ECU.
I don't want to dissuade you from buying and modifying a car, but I do want to help you learn as much as possible before you do it so that you know what you're getting into. The easy route is simply to buy a car that's already fast from the factory; it's what most people do. It's easier and will usually get you a car that's reliable and refined enough to drive on a daily basis, but still fast and fun when you want it.