now if you decided to go the diy route, go to PCpartpicker.com
there you can easily get comparison for price ranges on all the parts you'll need, and the added security of a compatibility mode, that only shows parts that will work with the pieces you already have.
when building a pc, you want to go with an Intel processor, i5 would do you just fine, just keep it as close to 4.0ghz as possible.
when deciding ram, you'll want at least 8GB for minimal and more of course is always a plus, up to what your mother board can handle of course.
also on ram, you'll want to keep in mind its speed/frequency/latency this will require a bit of research, but you can always just buy a compatible ram and not worry too much about this.
GPU I find as a rule of thumb, never get anything with an odd number in its name, like a GTX 1050 for example. I just never had any luck with them.
for faster loading times on OS startup and games, get a SSD (solid state drive) to install the operating system on. and then you can get a normal hard drive for all the other stuff.
so as an example, my first build, I scrapped a LOT from my HP pavilion that was made in 05 or 08 i cant remember. but I scrapped things like Ram, HDD, Disk drive, leaving the rest to be bought.
I started with the ram that was in my pc as a base as it was the main thing I had to compromise on and find compatibility with. my build was only at that point 350, that was Case, GPU, CUP, Fans, MOBO, Power supply.
My Gpu was bought off a friend when he built his new rig, and could do medium settings on most games easily. My final cost on a full build after replacing what needed to be, was 740, which includes a new HDD, and an SSD, new GPU and Ram.