From the looks of your hoppers they look like the larger hopper was added to the smaller hopper so they could load the hopper and would have to carry less bags of seed out to the field. I've always seen the dual hopper like this.
http://farmallcub.com/phpBB2/viewtopic.php?f=1&t=88435&p=703077&hilit=planting#p703077
As far as seen plates unless you already have a bunch of them as I do it might be better to get the corn and match the corn to the seed plate chart than get the plates you need. Some of the open pollinated corn may be ungraded. The open pollinated corn could be used for seed again but if you have hybreds in your area some cross contamination may take place so your next years crop might not be as good.
I can look to see want seed plate I used for my open pollination corn but may or may not fit your corn. If you send me a few seeds I will match up the best plate for it. It will be a better fit then looking at a chart.
I see by your photo you have the tractor attachment so you won't need anybody in the seat,which you don't have. I would bolt a piece of plywood there and add a bag of sand on top it would help the presser wheels cover the dirt.
If you are going to fertilize at the same time, if your bases are good the hoppers can be bought but won't be cheap. The stainless steel ones are really nice but a cheap alternative is to cut the bottom out of a 5 gallon bucket and use that for a hopper much like the ones that were on my planter.
In your hopper base check to make sure the knocker and pawls are rust free and operate as they should.
Unlike my sweet corn I've be planting my open pollination corn in rows of 18" which has workerd out well for me.
Any other questions you can post or PM me as you have been doing.
Kirk