I dont know your customer base or line up well, but it sounds like you have some older machines and are looking for a bit more compact/versatility.
If you are not a good mower mechanic and don't have the extra time to sit there and wrench.
check with your dealer and see if he's willing to take the old stuff in on trade and then buy some new stuff off his floor.
Since gravely is your choice.
A 52" rider and a 36" walk behind will do most residential stuff.
It's possible you might want to look at 32" or 34" units depending on gates in your area.
If you have bigger yards, get a sulky (wright makes the best one with the velke in my opinion)
The 36" with a velke can cut the heck out of smaller spaces like a front yard, while the 52" bangs out the back yards.
When the 36" is finished, guy parks it and picks up a weed whacker.
This combo is a tired and true platform of productivity I have been using for over a decade.
Trust me, Ill get a 36" WB any day over the 48" or 52" walk behinds or standers, unless I'm buying one machine for my main production unit (but you'll already have a 52" rider")
The walk behind will be more manuverable and get hills and ditches and other odd places the larger units and stand ons will not.
But my vote, listening to you, would be dump the old machines, and buy two new ones... Gravely 152 and a 36" WB with velke.