My first one that I bought during the panic back in 2013 was a complete rifle from a semi local builder, Warrior Arms. I did this to familiarize myself with one and gets some practical experience with it. After that, I realized how pretty simple it was to build. By building it, you can customize it any way you want the first time around instead of having to swap out all kinds of parts that will get left on the shelf.
I have built probably 10 lowers since then along with 2 of the uppers. I kind of like just buying the built uppers for simplicity and you can find all kinds of options to get the one that has the options you want. You have to realize that the AR's are designed around M4's that the military uses and have to be simple enough for an 18 year old to maintain.
Building lowers is definitely not complicated at all. The other reason I like to build my own lowers is because in Illinois, it's almost impossible to get approval for SBR's but AR pistols are just fine and dont need the $200 federal tax stamp and are less restrictive if you need to cross state borders.
You can take a stripped lower and build it into a pistol first then turn it into a carbine or rifle later but always have the option to go back to a pistol. If you buy a built rifle or carbine, you can never turn it into a pistol. It can only by converted to an SBR if you want to go with a shorter barrwl.