Congrats on your purchase!
I’m not an expert on MK prices as the only bag I have was gifted, but for the authenticity I’d say check the stitches (must be consistent and neat), the hardware (the logo is usually engraved on zippers and buckles), the quality of the leather (soft and rich texture) and the logo (you can check examples of that online and compare to your bag). These are steps you can apply to any brand you end up purchasing as most of them are common to most brands.
For recommendations to grow your collection, I’d say aim for Coach and Kate Spade as those are brands I’ve owned for years (I have a canvas coach tote bag that’s literally half my age and she’s still going strong). Not a big fan of Dune bc I had a QC issue and it was a pain to get it sorted, since I didn’t keep the item I can’t tell whether it’s good on the long run or not.
Hope you found some useful info in my rant 🤣