I love convertibles in real life, but I try to have at least three or four sedans or coupes on the layout for every convertible. Top-up convertibles aren't as conspicuous as if the tops are down. It's a substantial effort to find a good variety of period-appropriate closed cars and I've spent more money than I'd prefer on Brooklin and the like just because the less expensive models tend to be convertibles, sports cars, and luxury models. Kind of ironic that it's cheaper and easier to find a racing Ferrari than a Buick station wagon. (NEO makes a nice '57 Buick wagon, but it sells for at least $69.99.)
I'm mainly into steam era, which makes it even harder to find cars. About the only thing you can find in ordinary, working-class cars without spending Brooklin money is a '32-'34 Ford, and mostly what you find is a coupe or a roadster rather than a sedan. I'll probably pick up a Buick sedan from White Box; that's a nice one and reasonably priced. You can get reasonably priced 30's Packards and Cadillacs from several makers, but not Chevys.
I also model European trains and I've found quite a few nice 1930's European sedans.I just unpacked a four-door Citroen by Universal Hobbies that I picked up on the Bay for under 20 bucks, shipped. Often the Euro-prototype items go cheap on eBay because not too many buyers are looking for European sedans. It will look good next to the station when the MTH Orient Express pulls in.
Some mid-30's taxis. Chrysler Airflow by Rextoys, Ford by Solido, a different Ford by Rextoys. I think the Solido is actually a French Ford prototype, but it looks fine with American cars.
Model B Fords - Roadster by Ertl; pickup and sedan by Eligor. The sedan is excellent for a steam-era layout; I have a few of them in different colors. Not too expensive.
Three Packards by Solido. These can be found on eBay for around $20 plus shipping. The victoria also comes in a top-down version.