FME, the bike that one converts is whatever the individual wants to ride. My first conversion was an errand bike, and it turned out that a 20 mph front hub system on a mid-90's steel bike was perfect for the application. Next a steel hardtail with a mid-drive and seven years of off road use later it's been pretty great for that application. Etc as I've converted many more bikes for myself and others. The only problem with a cargo bike conversion (IMO) is that most have a flat area adjacent to the bottom bracket and with a mid-drive, ground clearance is reduced severely. This isn't a problem with OEM systems.