Hi there!
For a budget of 15k-20k, you can build a good entry-level PC that can handle basic tasks like browsing, productivity, and light gaming. Here are some recommended specs and parts:
CPU: AMD Ryzen 3 3100 or Intel Core i3-10100
Motherboard: ASRock B450M-HDV or MSI B460M-A Pro
RAM: 8GB DDR4 3200MHz (you can add more later)
Storage: 240GB SSD (for OS and programs) + 1TB HDD (for storage)
GPU: Nvidia GTX 1650 Super or AMD RX 570
PSU: 450W 80+ Bronze rated
Case: any budget case that fits your preference and size requirements
As for shops, Gilmore is a good place to start looking for parts. However, you can also check out other online stores like Lazada, Shopee, Dynaquest, and PCHub. Don't forget to compare prices and read reviews before making a purchase.
I hope this helps! Good luck with your PC build!