If the edge is a short cage it should be able to handle 28 teeth with a standard road double as this affects capacity while the biggest cog a derailleur can handle refers to it's physical limit which is determined by cage length.

As the spacing on Suntour and Shimano and Shimano compatible freewheels is slightly different you may find that the indexed Suntour does not play well with the new freewheel and Suntour and Shimano indexed components can sometimes be incompatible as well due to different geometries on the rear D that affects shifting.

Suntour freewheels can still be found on the used market and some shops (like mine) have the capacity to rebuild Suntour freewheels as we have managed to retain the service boards and cogs that were once common in many shops.