slowbrew/gymrat - I have done stereo equipment for my own vehicles since the mid 90's. Never in a professional fashion, but done enough that I am familiar with the components and the gotchas you should look out for. You have gotten some good advice here.
Crutchfield has a database of literally every car made and what fits where. as in what size speakers for the front doors, dash, kickpanels, rear seats, back deck, etc. As in what stereo receiver fits in the opening, and what wiring adapters you will need to connect them and what mounting kit you will need to convert the original oem stereo into a single DIN opening for your radio. A bunch of people will tell you to go to them for research and then go find the product numbers somewhere else and get it cheaper. For some folks, maybe ok, but for the do it yourselfer who is not 100% sure what to do, order from them and call the 1-800 number for customer support and they will help you overcome any installation issues you encounter in doing so. As long as you stick to the "what fits my car" in their system, they will support you in installing it.
Since you said JVC, I would say .. yikes. Really, decent stereo equipment is cheap these days, there is no need to go bottom of the barrel for a new stereo.
Since you have factory speakers, I doubt 50 watts is what you want anyway, more likely 20 watts RMS unless you intend to upgrade those. I would, but I would not go overboard, maybe some nice 30-40 dollar alpines in the 40 watt range. You do not need huge watt numbers to produce good sound. You need a receiver matched with speakers that work well together.
Give us the model and year of your vehicle and perhaps a target budget and I'm sure myself and or others can help dream up a list of things for you to consider. Slowbrew - you too.