Welcome!
An Officer is a fine costume to start out with. Not too hard, not too dear, and capable of trooping and wrangling at the same time.
And it has pockets!
Some off the shelf officers uniforms are fine, provided they are fitted correctly. (Cosplysky do a version for the 501st).
The belt can be bought locally from a member if you wish, and the boots can be sourced from most saddlery stores.
A good hat can be bought locally as stated, and I believe the guy doing the belts, can also do the hat and belt greeblie, (The discs).
The code cylinders are a little harder to get and do. You can mod chalk holders, get a cast version, or if you're lucky, and in the right place at the right time, occasionally the originals come up for sale.
The rank badge can be sourced locally too.
On a side note about the Mauser - The broomhandle mauser was changed to a DL-44, which had a couple of versions, as well as the Mer Sonn POwer 5, which is what the officers carry.
If you want to do your approval with a sidearm, it will need to be the Power 5, not a DL-44, which is the kits that Doopys sell, so you'll need to mod it a little.