Firstly, let me start by saying that I will not lecture you or tell you what you need to be doing at 18 because it is not my place to do so. The fact that you want to learn by getting your hands dirty is wholesome and heartwarming. May your tribe increase! However, I must tell you that it is easy to get carried away at your age while driving a fast car and I say this from a place of experience. Therefore, I trust you to exercise utmost caution and restraint while driving.
Secondly, welcome to the world of tuning and modifications. I have recently dipped my toes into it and have even occasionally gotten my hands dirty.
Cars to stay away from:
Now I would have normally suggested something like a Polo GT TSI, because I have owned one, and I have spent countless hours modifying it. But it did not take me long to realize that the 1.2 TSI is actually not the best engine to work on. Identifying its flaws initially was quite fun, and rectifying them was quite a thrill. But it has its limitations and is severely restricted in terms of potential. Even the handling, while neutral, is not 'engaging' as such. Therefore, I would avoid the GT 1.2 TSI Polo.
The previous generation Honda Civic is also a car I would avoid because it is not exactly mod-friendly as you may have been led to believe, unless you are considering a K-Series engine swap. The 1.8l NA engine does not receive extensive modifications well, and this has been documented by Nikhilb2008. I urge you to read that thread as it provides fantastic insights.
Stay away from the Tata JTP twins as they are plagued with serious (ECU related) issues that are potentially not-rectifiable. Such a crying shame because the factory package otherwise is beautiful.
Also, stay clear from the Mercs and BMWs for now. You may have the financial muscle to maintain these type of cars but it's not just about costs. Older Mercs and BMWs in your budget will only sit in workshops awaiting parts and you will not enjoy your car as much if it's collecting dust waiting for spares. Further, consider experiencing the joy of owning and driving a FWD car before you take to RWD cars. There is still plenty of time and FWD cars are a good stepping stone.
Cars to consider:
I would strongly urge you to consider the older Ford Figo or the older Ford Fiesta as these are absolutely fantastic drivers' cars. Although Fords do have their limitations too as their powertrains are not exactly easy to work on, they are by far the most fun to drive cars in this range. In fact, if you have a seriously strong appetite for a project car, why don't you consider buying an old Figo petrol and swapping out the engine and drivetrain with one from its sedan sibling, like a Fiesta 1.6 petrol? Nothing teaches you about engines like a good ol' engine swap. Again, you must have the appetite for it or such a project will destroy you.
The Mk-1 Octavia Rider / 2.0T vRS are great cars to learn from as well, and they are fantastic drivers' cars. But finding a clean one is extremely hard. So if you can't find one of those, I would suggest looking for a Laura 1.8 TSI. These cars have plenty of inherit issues that require skilled and experienced hands and the scope for learning is huge.
Alternatively, you could consider the Punto Abarth. There are a few used examples for sale I think. These 1.4 turbocharged engines and drivetrains are fun to work on and are factory-flawed enough to pique your interest. Do drive one though and see if it works for you, as it's not easy to get used to the awkward driving position.
Apart from the above options, there are the usual suspects like the old Baleno sedan, the old Esteem, old Honda City, Mitsubishi Lancers/Cedias and of course, the erstwhile jellybean Zen. These cars are tremendously easy to work on and won't break your bank like the VAG cars would.
Something to ponder about:
But before you embark on this project-car journey, the most important thing to do, and I cannot stress on this enough, is to find a competent mechanic/engineer who is willing to hand-hold you through your journey. There are several scam-artists out there who could very easily take you for a ride. Even though the tuning scene is still at its nascent stage in India, the dark underbelly of the tuning world very much exists and can easily destroy your enthusiasm and resolve. Therefore, it is extremely important to read as much as you can. Team BHP has a wealth of information in this regard and it will certainly help you make the right choices; but again, it is very important you read all the threads you can find before you make your decision. Feel free to seek clarifications or post your queries here, and don't hesitate to PM any of us if necessary.
I wish you all the very best with your journey and hope to see a fantastic thread on your project/build in the not-so-distant future.