I'd get all the parts from Weber in a single order as they should only hit you with a single $7 shipping charge.
here's a list of the parts you will need from them.
Qty Part # Product Name
1 85740 Top frame for Ranch Kettle, Painted $45.43
1 85750 Bottom frame for Ranch Kettle, Painted $45.73
1 85725 Axle for Ranch Kettle 32.37" , ~$10
2 6414 Locking caster, Genesis/Performer/Ranch/Platinum ~$5-6 ea.
1 987101 Replacement hubcaps (pack of 3) <$4.00
4 87417 Aluminum tubing plug for Ranch Kettle $5 ea., $20 for 4
An optional part would be the front handle of the RK. I haven't seen anyone else put one on, but I will be as it should add needed stability to my build. I'm attempting to do a complete "drop in" solution where I won't have to drill any holes into the kettle itself. It's proving to be a bit tricky :-)
Anyways, the front handle is part #85730.
I should add that just about none of the parts above include "hardware", i.e. nuts, washers or bolts. It's your choice whether you want to get them from Weber or from the local hardware store, the latter being the best option if you want to go with stainless steel hardware.
Here are those parts:
87410 1 Hex head screw, 1/4-20 x 3 inch, black < ---- Center "pin" joining the two lower frame sections.
4 4086938 Hexhead bolt 1/4-20 x 1 3/4 inch < ----- Bolts the front handle to the frame
5 87503 Hex nut 1/4-20 inch, zinc <---- fits all the bolts.
10 87403 1/4" Flat nylon washer, black < ------ used on all the bolts
(If you don't do the front handle, then you only need 1 hex nut and 2 washers.
Lastly, when you call Weber, do some social engineering and chit chat with the CSR a bit. When you get around to having them look up parts, start with the higher priced ones first, the top and bottom frame legs. When they give you a price (which should be about what I listed, ask them if that's the best they can do. You might get lucky.