I’m going to be pedantic and awful for a minute. I’m sorry in advance.
“Button down” shirts are shirts that have collars who’s points button down. A “dress shirt” is a shirt where the entire front opens with buttons. The terms are not interchangeable.
Honestly, all you really need is a stack of Oxford cloth button down shirts. Light blue mostly, but other colors are ok. They look great and wear well either ironed and crisp or rumpled and relaxed. Roll the sleeves up when it’s hot- short sleeved dress shirts are an abomination.
Wear a tie. Leave it loose with the collar button undone if you like, but if circumstances call for it all you have to do is button up and you’re a whole new level of class.
By pants that fit. Pleated or flat front, but get the right length. Have them hemmed if they need to be.
Me? Oh, I’m glad you asked.
I wear double-front duck canvas pants almost every day (Taylor Stitch Camp Pants). I wear a Oxford cloth button down shirt with a tie almost every day (shirts are often Lands End Sailrigger, but are sometimes bespoke. I’m spoiled). I wear leather shoes (often a pair of 60’s Florsheim Imperial Longeings in Shell Cordovan, but sometimes a pair of Jeans Shop pigskin captors).
This is my uniform, essentially. Easy, versatile, and works well for me.
Your mileage may vary. Etc.
[edit because i can’t type so good]