There is a ton of info on YouTube on how to remove and install the radio on a 97 era Camry which is really nice if you want to do it yourself.
For example:
For an actual head unit, you will want to figure out what features you want for it. Most don't have internal storage. That would be a bit of a headache. But many DO have USB ports and you can pick up a micro USB drive which can store 256GB or more of songs onto it for about 20 bucks...
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MANY head units these days feature Android Auto or Apple Car Play. I've not used either feature, but many people do and as time goes on, this will become a more and more important feature. So, that's worth consideration.
Obviously, a larger dual din head unit with a screen will cost a fair bit more.
IMO, head units tend to be decent, but not great, and listening in a car itself is just a lousy way to listen to music. But, speakers tend to be far more important than the head unit. But, I will buy and spend a bit more on a head unit that I like. One that has the features I want and is easy to use rather than get a head unit that doesn't work well or isn't something I find ergnomic.
As such, I will go into Best Buy or any other local car audio store, and spend some time playing with their head units to see what is decent.
If you read reviews of head units on Crutchfield, it is pretty easy to see how the cheapest models, like those from Jensen and Boss still hold up VERY well next to the more expensive models from Sony, Pioneer, and Kenwood.
I would literally get any model that is in the $100 price range and call it done as long as it has USB playback and Bluetooth in it, then I would pair it with any set of decent speakers. I lean towards the Infinity speakers myself. Typically I have spent more and gotten their Kappa speakers, but honestly, they are well reviewed and when I got a cheap set once, they worked just fine. It's easy to see how you can adjust budget all over the place to make things work for you as you like. That is, if you find your rear speakers on decent, it opens up more money for good front speakers. If all your speakers are shot, but the receiver you really like is $200, then there are halfway decent speakers for $50 a pair. Infinity starts at about $70 a pair, so it would require a bit more budget or to lower the quality of the head unit.
I know, several years ago, I had a Jeep that had all the speakers in it that were worn out. Probably cranked it up and blew them all at some point, then they just rattled down to nothing. I hit up Crutchfield and ordered replacement speakers and the new ones weren't great, but were a solidly good and a vast improvement on what I had in there before. So, it doesn't really cost a ton to get decent performance.
I put a new head unit in my 2004 Odyssey, and I just had Best Buy take care of it. They had free installation and I only paid $120 for a Bluetooth and USB head unit that still had a CD player built in. Works perfectly fine and was nice to add Bluetooth and USB playback to my car. Bluetooth is really nice for the phone.