Find a female friend with a good sense of fashion and get her to spend a day with you going to some department stores and men’s clothing shops to try stuff on. She’ll be a great critic and you’ll be able to get a sense for what types of clothes fit you, if there are colors that suit you well, etc. Take some notes so you don’t forget your dress shirt size or whatever a few months later. Buy your friend a nice lunch and thank her copiously.
Colors and fits are highly individual; you may find that a regular cut dress shirt fits you great or that you need to get something special to look good. You say that you’re overweight so big & tall stores would be worth checking out - I’ve known guys who were pretty hefty but they carried their weight well and wore clothes that fit them right and they looked mighty sharp, not like some dude who just got draped in a random 2XL.
Personally I hate khakis. They’re just too blah. Skip the khakis plus pale shirt plus navy blazer ensemble. A friend of mine calls it the Prep School Hell look. You can get brown slacks that look much better IMHO.
I think GilaB is right on option 3; dark slacks go great with a polo shirt as well as a dress shirt. Pick up a couple of ties (and learn how to make a nice knot!)…a lot of guys go with some pretty basic color palettes in the shirt & pants department and a nice tie gives you the opportunity for a splash of color. Clean dark jeans also go well with nice shirts and you can still add a tie to dress it up. That will cover your range from “casual” to just below “dressy” (I’d call matching slacks and jacket, shirt and tie to be “dressy” - your basic job interview outfit).
I’m not a fan of white shirts because they’re tough to keep clean - any little spot or stain, dirty collar, etc. is pretty blatant. Ditto with undershirts - Jockey sells packs of nice cotton undershirts in colors.
Taomist’s suggestion of a sport coat is good if you think you’ll be doing stuff where something more than just a dress shirt is needed. Some kind of warm sweater or jacket that you can wear with a dressier outfit is nice - doesn’t have to be pricey, I’ve seen pretty nice dark wool peacoats and similar for $50-$100 on sale. Good on a date where it’s chilly outside.
Almost none of this stuff will look good with sneakers, get some comfy casual-dress shoes. A good pair will last many years and can be resoled (I finally wore out a great pair of Eccos after about 15 years). Rule of thumb is that all the leather stuff should match - if you’re wearing black shoes have a black belt.
Random thought - if you think there are a couple of celebrities who look like you (either their face or their body) search for online images of them - you’ll probably find a bunch of shots from talkshow interviews, awards ceremonies and whatnot where they are dressing up a bit and you may get some good ideas. Here’s a couple of shots of Zach Whatsisname:
Dressed up (warning, flipping off the camera):
Informal but not sloppy:
If you’ve got a beard, keep it trimmed neatly. Whenever I see a raggedy dark beard on a guy it always makes me think of this dude: