$5K is a fantastic deal - go buy it now, if it runs well and makes no weird noises. Then do homework. Watch all the videos you can, especially taking off the panels, where the battery is to check it, all the how-to's etc. Couple things to do:
1) Order an oil change kit from LaMonster.com. Change the oil & filters regardless!
2) Check if the battery is original Yausa. I'd replace it if so, they like to go bad quickly. I get mine at Batteries Plus. Never had an issue.
3) Look at the front sprocket, if it looks like it has red dust on it, you should pull it off and inspect the inside teeth. The pulleys go bad - if you are lucky you will feel & hear chattering when you accelerate and make it home before the pulley goes flying off into a cornfield. I never found mine

It isn't hard to replace, plenty of great videos and lots of threads here about it. Mine lasted to 30K miles on both my RSS's.
4) If you are going to work on it yourself, look up replacing the vacuum lines (3mm ID silicone tubing) they will get cracks and cause problems
5) Think about a plug and wire kit from Baja Ron (on EBay)
6) Order a set of brake pads to have on hand (amazon) or replace if less than 1mm thick (lots of videos, easy to do); flush brake fluid.
7) Speaking of brake fluid, keep both topped off right to the bottom of the threads of the caps. Even a couple teaspoons low will throw a code
8) Learn how to pull codes - remember if you see anything (check engine, or blinking things) do NOT shut off the bike, get codes before you do. They are stored and cannot be retrieved after you shut down. Only a dealer or someone with the hardware can pull then.
9) Read the threads on tires. If you have original Kenda's they will not last long (10K if you are lucky). Most of us use car tires.
10) You can get great deals on factory add ons, being the bike is old-ish. I picked up nice BRP rims that were $550 in the day for $115 shipped a couple months ago!
11) Add power to the dash. Several good ways to get USB power up there.
12) Phone holder I use:
https://www.amazon.com/Metal-Motorc...RXY02GTSM8M5&qid=1573554282&tag=cruiser_os-20 (I think this is the one).
13) The 998 VTwin sounds AWESOME with an aftermarket muffler. Super easy to put on. I have an HMF muffler, nice rumble. Straight pipes are way loud.
14) wrapmyspyder.com has all kinds of cool stuff and wraps to customize!
You will have SO MUCH fun accessorizing, adding LEDs, bumpskid with LEDs, on and on!!