My suggestion is to pay the car off and then worry about mods. After you pay it off, and if you plan to keep the car a long time (i.e 10 years or more) then mod it however you want. But it seems to me that people that buy used cars prefer to buy them in stock condition. So if you think you might end up selling before 10 years, keep it base. Otherwise, do whatever you want to it, within reason.
I would hope that goes without saying - I don't see anything in the OP that mentioned carrying a loan/lien on the car.
But hey kids: in general, if there is a lien on the car, it's not really yours, you're borrowing it from the bank.
That may sound like a trite saying, but the fact is that modifying the car generally REDUCES not increases its value, so don't do it (except superficially) until it's actually YOUR car, completely, or you could be in for some pain.
Most or all car dealerships would not only not give you anything extra for your lowering springs, aftermarket intake and exhaust, etc., but will actually
discount their offer (significantly) because they'd have to remove them and restore the car to stock in order to sell it. (Surprise! Buying a modified car from a dealership is Not Really A Thing).
And so, if you modify your car in a way that you can't easily reverse, you're most likely going to be looking to find a private buyer who likes or doesn't mind your modifications when you sell your car. For max value, you would be looking for a fellow "enthusiast" whose tastes run similar, and would have done what you did anyway, or will mod it themselves anyway and doesn't care.
But you can't sell your car privately without a clean/clear title for immediate transfer. Nobody buys a car privately that has a lien on it: you'd have to pay off the lien (with the money they just gave you to buy it) to get sole title before transferring it to the new owner, which would take a few weeks.
If you had to suddenly sell or trade in the car before you paid it off, those types of mods will be a double burden - you paid for them once, and you'll pay for them again in lost value.
Tint the windows? Sure, almost everyone does anyway (and it's easy to remove). Add a powered subwoofer to the stock audio system? Relatively easy to reverse. Tune the engine, even? You can restore the ECU to stock. Change the wheels? Keep the OEM ones in storage to put back on until you're clear.
Modify the exhaust / cat delete / headers / lowering springs / sway bars...? Those are all negative value mods for resale.
