If you’re just replacing the stereo base (i.e., not doing the speakers or anything), it’s not too difficult to do it yourself - mostly, it’s a matter of matching up wires. The wiring harness makes things a lot easier, and also makes it easier to replace the stereo down the line, but if you can follow a diagram, it’s not too hard.
In the price range you’re looking at, it probably doesn’t matter too much which model you buy, because you’re getting a low-end, basic model. I’ve pretty much gone with the cheapest CD player I could find for the last couple times I’ve done this, and never had any real problems with the unit.
My only caution is if the former owner’s brother-in-law, Benedicto, did the installation of the previous stereo, you should put down the money for a professional installation. Benedicto strongly subscribes to the “it doesn’t matter if you use the right wire color to connect things, because it’s all going to be wrapped in a big ball of electrical tape anyway” school of thought, which is a school that doesn’t lend itself to easy removal.