It really depends on your figure. If you stoop, are corpulent or over erect etc, an off the peg suit will be a poor fit and more or less impossible to adjust to you. E.G. I had a young bloke come in recently for a suit who would never be able to buy ready to wear and get a good result. He had sloping shoulders, a round back and his right shoulder end was 2" lower than his left. The pattern I cut, someone altering a rtw suit would never get to because there aren't any allowances inside the garments to get the necessary shapes to the panels.
Honestly, we dress differently these days and the suit market has totally changed. No one really knows what a good suit looks like (even the folks making them) and the way They are cut, most rtw suits are massively compromised anyway. Get something that you like that isn't going to need huge alteration and go with that. Altering rtw clothes a lot is not possible without creating distortion and don't let some salesman tell you it can be done. They either don't understand or they are lying.