What's the definition of "impatient" in your mind?
I'd say if you feel like grinding is necessary, then you may be impatient. For me, I'm fine using a small fraction of cars for various events in World Circuits, and for Sport Mode, I always rent. I've been playing since the first month of launch or so, and I'm up to 69% of cars collected. (I even have all the invite-only cars, plus a few spares I won via random tickets like a spare Ferrari FXX K and a spare Aston Martin Vulcan.) I think it also helps that I rarely buy more than one of the same car. I also save tons of credits only buying parts for cars when I feel like it's absolutely necessary, including engine swaps and widebody kits. Not to mention all the goodies you get by getting gold on all the currently-available missions, circuit experiences, licenses (including the Master licenses), Bonus Menus, Extra Menus, etc. as well as just the "base" menus.
For what it's worth, there is only one engine swap that I'd especially recommend buying if you don't have a spare engine laying around: the GR Yaris with the 2JZ. It's amazing, especially with a torque splitter set to 50:50. You can use it in pretty much any event that requires road cars that have up to 700PP.
For the actual WTC 700, though, I just use the Ford Mustang Gr.4 that you get for free through the missions. Similarly, I use the Chevrolet Corvette C7 Gr.3 for the WTC 800 - another car that you get via the missions - and then the Hyundai N2025 VGT (Gr.1) for the Gr.1 Prototype Series. As for the WTC 900, any Gr.2 car will do, but I usually use the Mercedes-Benz CLK-LM, which also happens to be what I use for the Neo-Classic Competition.
EDIT: I also am willing to be patient in waiting for LCD cars that only come in a single color/livery, which would include a lot of them since many cars available there are historic race cars. It's the ones that come in a larger variety of colors - like the McLaren F1 or the Plymouth Superbird - where I might jump on buying one as soon as they appear in a color that I particularly like. Whereas a car such as the Porsche 550 Spyder is one where I'm willing to wait notably longer since it only comes in a single color, and I'm willing to wait even if it's needed for an Extra Menu.