Mileage?

sounds like you don't know a lot
let's start with the basic maintenance from the book and then let us know once all those are checked

Let me help you with your questions.
TPMS optional item, if dont want every week pump air je. Like your weekly routine.
Voltmeter is definitely a must as you need to look out for your battery health.
If battery very weak dy, you might overwork your rectifier. Rectifier kaput then your cost of fixing goes up.
For fork/shock change, upgrade is optional. See how your bike performs according to your riding style. If your bike feels solid while riding, good nothing to change.
Aftermarket options allow for adjustable rebound/compression, therefore finetuning/improving your riding experience.

Those are definitely important. Do not skip on safety gears.
Helmet try getting the ones with ece22.06 rating.
Jacket, pants,gloves, boots you can get benkia/komine/rs taichi for asian fitting.
Also dont forget rain coat, since raining season is almost here

Depends on your budget

High-end gears can cost thousands like Alpinestars, Dainese, HJC, AGV, Arai

on a budget can look at Star Field Knight, Komine, KYT, RCB

better try fitment first at the physical store before buying like BikEmpire96, Serdang motorcycle, Bikeworld

decent helmet options.. hjc is a much bigger brand. c10 is entry level iirc, not bad...  same like jackets and gloves, go try em out. assuming they feel the same, id take the c10
generally ppl have voltmeters on the r25 to monitor the charge voltage, cause this bike commonly has the error 46 issue typically caused by the rectifier socket.
typically the other issues arethe valve sopek issue (mostly down to poor maintnance) and the idle gear perishing (resulting in no oil pressure, also down to poor maintenance)  but yours so new, these 2 shouldn't be an issue.
haha do update us on your quest for tires... got to change mine too. myself am considering maxxis s98 and irc S99... but ill be frank im fairly new to all this...

Riding gear with budget <700 won't have abrasion resistant.. those are found on higher end gear.
Even so they are good enough, check my review on an accident I had while wearing komine jacket, rs taichi gloves and benkia pants
https://forum.lowyat.net/index.php?showtopi...#entry109075491

Michelin or Pirelli, both have great grip and durability.

which Pirelli?

Since its a used bike it could be because of wear and tear.
Check if handlebar screws are properly tightened.
Handlebar grip might also need to be replaced something with softer compound like Domino A010.
Gloves with thick fabric at the palm and finger area could help too.

Other factor could be that you are gripping the handlebar too tight. This is common to new rider.
Loosen the grip and make it that you can turn the handlebar with just your fingertips.
Relax your shoulders so that you arent pressuring your wrist.

R25 doesn't vibrate from my experience