Over the years I've had dozens of guitars, new and used, and I think there may have been maybe two or three that didn't need anything other than a string change and a good cleaning/detail. Most of them needed something (or some things) to make them right for my style of playing.
Here's how you know:
If you get the guitar and it sounds great, tunes up easily and stays in tune while playing, no buzzes or rattles anywhere, and it feels so good playing it that you don't want to put it down and it puts a huge smile on your face and you find yourself doing things you've never done before because you're having so much fun playing this amazing guitar... it needs nothing, and you lucked out and got a great guitar! I've had maybe two like that EVER.
If it doesn't do that, get a good luthier/tech and make it happen, or if it can't be made to play that way, return it or sell it! You only become a better guitarist if you actually PLAY the guitar...a LOT!
And you need to shop for a good luthier/ tech like you would shop for a doctor. Seriously. Get referrals, check Yelp, see if they have a website, ask around. And never just go for the cheapest guy! There are some guys doing this who shouldn't be allowed near guitars! And there are some who are true luthiers, and stand behind their work and can take a guitar you thought was alright and make it play like OMG!!! And you might go through a few guys til you find the right one, but it is so worth it when you do!
Don't be shy, be prepared to play for your luthier/tech. He needs to see what you're doing and why, and to make the adjustments for you!
At some point, you might get into needing different setups for different guitars... different tunings, slide, one for hard strumming, one for delicate finger-picking... whatever. That's why you see some players with a few different guitars during a set. Sometimes they're not just showing off, there's usually a reason that they are lugging so many guitars around! And you might end up buying one that was simply set up for a particular preference and it makes it hard for you to play. Your guy should be able to fix that!
Ultimately, you want your guitar/s to be the absolute best they can be. So that you can play effortlessly for hours on end as you are discovering new things about your playing.