Forget the down rigger, bought one used it for a while then sold it for a better method!!!
we now use snapper leads and heavy tackle rubber bands
pull 8-10m of line off with your bait. then put the rubber band on the snapper lead of choice (current pending) we use up t0 16oz then wrap the band 5-6 times around the line and put the lead back through the loop of the band. Try varying heights. Engine just in gear we usually run one 20ft off the bottom one mid water and one on the surface
we get back 9 out of 10 sinkers, just break the band when the fish is close to the boat.
You can spend a lot on sinkers and bands before you get to the cost of a downrigger. Also price what a downrigger bomb and re attachment kit will cost as you WILL loose a few bombs