I have bushings on my walk-behind mower and after 16 years of dealing with trying to keep them lubed I would never again buy a mower without ball bearings in the caster swivel. But I'm a professional running at full capacity trying to keep a couple dozen pieces of equipment up and running so I'm a bit short on time and patience, lol. With a ball bearing setup such as on my zero turns they're virtually maintenance free. Just something you should be aware of.
The Hustler Raptor (same parent company as BigDog) is pretty popular as a residential mower and I think it fits your budget. The SD seems to be pretty close to being a commercial unit. Plenty here about them if you do a search.