DESKTOP SYSTEMS are usually a little more flexible, upgrade-wise, than laptops. There’s generally a lot more room to look around, which means a lot more space for dust to gather, too. RAM and SSD upgrades will probably, again, have the biggest impact, but you may also be tempted to replace your processor. If you want to get into gaming, you might want to put in new graphics card. Heck, you might want to rip out all the guts and replace the motherboard and everything else mentioned hitherto.
There certainly are bargain components to be had on auction sites and marketplaces, but these may be damaged and unscrupulous sellers will not refund you. Bear in mind also that components have a limitedto life, but older machines aren’t as good at saving power when idle as their newer counterparts.