If you're powering speakers with the amp built into a radio head unit your speaker budget shouldn't need to be more than $100. Having expensive speakers powered by a head unit is like putting 1000hp in a bicycle.

If you have the money to spare, you'll really benefit from having a separate amplifier, in which case buy the best speakers your budget can withstand.

You mentioned crossovers, any component set will include them along with tweeters and the midrange. The "thing" to do these days is get a DSP and use 1 amp channel for each speaker/tweeter, eliminating crossovers. This allows significantly better tuning flexibility.

If you're looking for quality sound, the first step is a clean signal, like CDs or FLAC files.

you need to take a caliper an measure your tweeter locations yourself. all tweeters will have dimension specs available online. you'll likely have to do some mild fabrication no matter what size tweeter you choose. a general rule of thumb is bigger tweeters play lower and thus sound better in 2-way front stages but smaller tweeters are easier to fit and install.

The 6.5 vs 6.75 shouldn't make a difference for install but bigger is generally going to play lower in the bass spectrum. depth is usually the most important dimension to pay attention to for the midrange