Most stems today use bars that are 31.8 but the specs for yours say 25.4. Look where the bars go into the stem, if there is a spacer then you can use either 25.4 or 31.8 diameter bars. I would hope it is a spacer because it will be easier to find inexpensive 31.8 bars and you will find a larger selection especially in black to match the bike. Just take out the spacer and install the 31.8. If it is truly a 25.4, then you need to find that size of bar to fit.
Next I would buy a pair of drop brake levers for caliper brakes, Tektro or Shimano are two names that come to mind. Make sure they say this on the box. Then add a front brake. Most of your braking, about 80%, is actually done by the front brake. You will need brake housing and a cable for the front brake. Last buy some bar tape and follow a YouTube video on how this is done.
The upgrade is realistically going to be about $80 and that is if you do the work yourself and find inexpensive parts.
Personally I would leave the bike alone as I love flat bars. The only thing I would do is add a front brake. Enjoy the bike as it is and if you have the itch to buy something, buy a good lock or back pack or courier bag to carry your stuff. Have fun.