Welcome to the wonderful world of the XS650.
On your carbs, rebuild kits are seldom needed. A kit has some gaskets that work, some have float valves that work. If they come with any other parts, those seldom work.
Better to tear down, clean inspect before you buy anything. The stock gaskets are fairly robust and can be reused. The brass parts can be cleaned and used.
Often cheaper to buy just the parts you need rather than the kits.
On the ID, check the engine, on the front kinda behind the front engine mount on the upper case half. Is a number.
From the factory that number matched the number stamped into the steering neck. If it does fine, If it doesn't that's fine too.
There are charts that you use those numbers to determine the year and model of your bike. Most states will use the frame number for title and registration.
Up near the top of the Page you will find a black bar, Click on the TECH. This takes you to a list of categories. Under each category there will be more specific listings. Search the tech section and you will find answers to most of your questions.
For those that you can't find answers too then ask here.
The places you found for parts, most are pretty good, Mike's is a so-so place. Some good parts, others not so much.
There are many threads talking about Mike's quality issues.
Leo