There are many things to be said regarding this post but I will try to be as short and sweet as possible, The main piece of information that's missing is where you are getting these clubs. I know you said you were getting used clubs, so are we talking ebay, marketplace, etc? The reason I need to know this is because if your fitting turns out that you may require a different shaft flex or whatever, it's going to cost you more in the long run to retrofit the used irons. The whole purpose of getting fit is to go in with an open mind and let the fitter determine what's best for you based on the data he gets from going through the fitting (and a bit of your feedback).
If you can shoot in the 90's, you would definitely benefit from getting fit. Here is the thing, if you are reasonably happy with your level of play, then once you get used to the new clubs, you should see a few strokes come off your score. If you plan on taking lessons, it's unlikely that the fitting would change all that much so I wouldnt worry about that. If anything, it could be a little lie adjustment or something like that, but you wouldnt need a whole new shaft.
All that being said, at 5'9, my guess is that you're very average in terms of a standard club length and lie. The main thing you want to get right is shaft flex and weight, which is mostly related to how fast you swing the club (there are other factors but too much to get into)
For background, I have been in the golf industry for almost 30 years (still am) and at one point was a fitter and a builder of clubs.
If you can shoot in the 90's, you would definitely benefit from getting fit. Here is the thing, if you are reasonably happy with your level of play, then once you get used to the new clubs, you should see a few strokes come off your score. If you plan on taking lessons, it's unlikely that the fitting would change all that much so I wouldnt worry about that. If anything, it could be a little lie adjustment or something like that, but you wouldnt need a whole new shaft.
All that being said, at 5'9, my guess is that you're very average in terms of a standard club length and lie. The main thing you want to get right is shaft flex and weight, which is mostly related to how fast you swing the club (there are other factors but too much to get into)
For background, I have been in the golf industry for almost 30 years (still am) and at one point was a fitter and a builder of clubs.