I noticed that you referred to the look of the powdercoat post-baking as "plastic-like"... well, that's because it's plastic. That's why you have to bake it. The normal powder doesn't actually completely coat the surface, and even if it did, you'd already noticed that it doesn't adhere very well... when you bake it, the particles of plastic-material "dust" melt together, bonding to each other and to the part, making a very durable colored finish.
Your primer job looks very good, but if you're going to paint with yellow paint, it's going to take quite a few coats to get a good even tone over gray... yellow primer (check a PPG place near you) might be a better option for you, for the best finish.
The powdercoat will likely not match the paint, so be prepared for that eventuality.
As far as the rims go, many older tractors (and some newer ones) came with hub adapters on them already. I have several sets that I use for other projects, but they can be had at nearly any tractor shop, and have a US-Spec 5x4.5" pattern, which can be found in 8, 9, 10, 12, 13, 14, 15, and 16 inch utility rims, as well as anything automotive up to 24" inches.
I often run truck tires on my tractors, because they're usually cheap/free, and that requires a modified 15" 5x4.5" rim with a truck tire mounted on it. Backspacing can be a concern, it's likely best that if you go this route, you get a wheel rim from a RWD vehicle, something with a deep "dish" to it.
The matching metric wheel pattern is 5x114.3 which can be found on several imports as well as many US vehicles, or 5x115 (GM specific, mid-size.)
For a quick starter list of vehicles with that bolt pattern -
Ford Ranger
Pontiac Grand Am/Grand Prix (all years FWD)
Ford Explorer
Ford Taurus (Not sure about anything after '99-ish)
Honda Accord 5 lug - possibly.
Those are among several others. The bolt pattern is one of the more common ones, and is also used on 5-lug trailers largely.
Here are some eBay auctions that will work:
Snapper LT16 Lawn Tractor Mower= 1-1/8 ID Back Axle Hub - eBay (item 250463639834 end time May-08-10 19:31:09 PDT)
Rear Wheel Hub for Wheel Horse / Toro Garden Tractor - eBay (item 120554530771 end time Apr-15-10 16:54:22 PDT)