The 2013 has the FB25 engine - the timing belt isn't marked because that engine uses a timing chain. I'd recommend replacing the serpentine belt, replacing the spark plugs, then depending on how often you've been doing it, I'd replace the fluids - gear oil in the front and rear differential, CVT fluid, brake fluid, and oil, along with the cabin and engine air filters. Then while doing that, look over the brake lines, brake pads and rotors, etc. and replace anything that needs replacing.

 

+1 To this list -
I'd also recommend replacing the idler pulleys on the serpentine belt, and maybe the serpentine tensioner as well. I tend to replace them at ~100k because I've thrown too many belts in my years. When I did the idlers on my 2012, it was clear they were very worn and while they weren't free spinning or worn enough to have some wiggle in them, they did spin much more freely than I felt comfortable with.

 

are there any videos or DYI guides for belts and pulleys and tensioners replacement? pls
Any recommended kit for this drive belt replacement work?

 

Replacing the serpentine belt is pretty straightforward. I usually use OEM, but Continental or similar should be just fine (it's an inexpensive part, so not a ton of money to be saved going aftermarket)

For videos of the install, you can find just about whatever on youtube - here's several videos showing replacement of the belt and one with the tensioner for an FB25 in a Legacy (identical to the Outback)

 

It's time for me to do this replacement,
Would you recommend any kit for the belt with tensioners and pulleys?
Otherwise if to buy each one, what to buy and from where?
thanks.