DIERYA has some good budget keyboards, when I started getting involved in more competitive games they were my first mechanical keyboard and they lasted me some 3-4 years before I upgraded. Never had a problem with switches or anything of the sorts.
RK Royal has some good mid tier keyboards as well, and a few of them sound pretty nice too.
Give some thought before buying a tablet, just because a lot of people use them doesn't mean its necessarily the right choice for you. When I started playing I saw everyone was using tablets so I quickly invested in one, tried it for a few days, and absolutely hated it. I think a good way to tell if a tablet would be a good idea would be to just watch some maps played on auto, holding a pen over paper, and just pretending like its you that's playing. If you think it feels comfortable, then buy a tablet, if you think that sticking to a mouse might be the better choice for you, then I can give a few recommendations.
As far as mice go, I think if you actually want to be the best YOU can be at osu, look at mice with a 1000hz+ polling rate. 125, 250, and (sometimes) 500hz all have issues where if you need to aim fast, you can lose out on a bit of input and 1000hz is where you see the issue become virtually non existent.
DeLUX M800 is a very good mid tier mouse that has a shell that is more or less a perfect copy of the Razer Viper mice. It is wireless too so you wont have a pesky cord dragging across your desk all the time. (don't worry, wireless mice input speeds are practically identical to their wired counterparts now)
If you like Logitech, check out their G305 Lightspeed. Again a very good budget mid tier mouse that is wireless as well.
You could also look at buying higher end second hand parts from local selling services. I'm not sure what services like that are available in Vietnam, but in the US things such as Facebook Marketplace, Ebay, Craigslist, and Offerup are where you would typically find things like that, maybe a peek into local alternatives for you could be worth your time and money.