The main advantage a laptop has over a desktop is the portability. That's pretty much it. If you don't need the portability, then a desktop would most likely be better. As a matter of fact, desktop PCs are better to work on in case you ever want to upgrade it or fix it.
I'll give you a brief description of the major parts of a PC:
The CPU is the "brain" of the computer. It does the majority of processing that goes on in regular usage. There are 2 main brands: Intel and AMD. Both companies offer CPUs at different price points, capabilities, specs, etc
RAM is Random Access Memory. It holds your programs when you open them. Your programs are loaded from disk into RAM where they can be worked on. The more RAM your PC has, the more programs you can have open. Most typical computers have anywhere from 6-16 GB of RAM. For your use cases, I would go with at least 8. Having more RAM doesn't hurt, but if you're not going to be doing anything memory intensive, it doesn't really make sense to have too much.
Storage. There are 2 main kinds of storage in computers: Hard drive and solid state drives. Both do the same thing in that they store your data. However, SSDs are much faster than hard drives. They're flash-based storage so they can read/write data much faster than a hard drive, which has mechanical parts. However, SSDs are currently more expensive per gigabyte than hard drives (though prices are always falling).
The advantage a hard drive has is that they're pretty cheap and have high capacities (many terabytes). Most desktops have a hard drive. They're ok for average use, but I would highly recommend getting a system that has an SSD for the OS and a hard drive for general data storage. Things like movies, music, pictures, etc won't sound or look any better if they're on SSD, so it doesn't make much sense to store them on the SSD.
As far as security software goes, you don't have to buy any. Windows 10 computers come with Windows Defender. It isn't the best thing in the world, but it has improved a lot since it came out. I personally use ESET and it works pretty well and is lightweight.
I'm not sure how much you're wanting to spend, so I can't really give you any recommendations.