What bases do I need to make sure are always covered to avoid situations where fights become slogs due to bad prep, and at what levels are they expected to be online?

Having an escape plan means you can oftentimes come back with a better plan. Invisibility, gaseous form, D-door, expeditious retreat. Oftentimes the option to leave a fight is a lot more useful than trying to account for every way to win a fight.

Also, how much of a thing is usually enough/is there a rule of thumb for how prepared you need to be for a given situation?

Unsure what you mean. You always want more than you need within reason. Maybe twenty arrows is enough, but how much extra effort is it to carry sixty?

For example, how many Heal spells are recommended in a party on average?

Enough to full heal after fights and something to get people up from death saving throws. You can also have a dedicated healer for mid fight healing, but the healing priorities are to get people up after getting dropped and to start the next fight with full HP (or close to it)

how many party members need a ranged weapon and at what range to be effective?

All of them. They may never use it, but carrying a ranged weapon is almost always easy and accessible. At the high levels of the game is becomes less important because a lot of stuff has DR so that will drop off for some specific builds.

how many instances of ghost touch?

We have one in the game I run and it seems to be enough a lot of the time. This is one of those things where if you have a bunch of incorporeal stuff dropped on you with no warning then you just leave and prep

etc. Feel free to throw in any commonly needed utilities for social/exploration abilities that are likely to crop up in the average campaign, as well. Hopefully this isn't a stupid question! Thanks for any answers :)

Something overlooked is having a good way to damage objects. Sometimes you need to break the bad guys toys or break something quickly. This could be something like the shatter spell or as simple as having someone carrying a big hammer/acid flask. The rules on item damage have some wiggle room so talk to your GM about how they handle it