I nearly always build my freight cars because I like to build the kits (as well as buildings). I’ve found some old Athearn blue box kits at train shows but mainly have settled on Accurail and Bowser kits, buying about one of each type in 40’ or less cars (transition era and earlier). I’m quite satisfied with both brands. There are others that take more patience and have more detail and may not cost any more (e.g., Tichy flatcar) or a few dollars more (e.g., Branchline or some Proto 2000 still out there). I’d suggest try a Bowser and an Accurail and see what you think. If you like the kit building, try others, particularly for a certain car type you want.
I add Kadee couplers and metal wheelsets (wheels and axle, using the stock frame).
Enjoy the hobby.
P.S. - here’s a prior thread on folks’ comments on different brands of rolling stock that may be of interest.