Don't wear a polo shirt. You aren't going to a "mixer" at your fucking fraternity, you douchebag. Wear a black suit with no accessories, except for an understated watch if you have one. Leave the big, flashy Rolex at home.
Do NOT wear a black dress shirt. Maybe it's just me, but I've never liked that look with a black or dark-colored suit. If you have a charcoal grey shirt, wear it. Otherwise, white will suffice. Wear a black tie if you have one, otherwise just wear something with a really dark, simple pattern. Use an Oriental or half-Windsor knot for something very simple. No pocket squares/handkerchiefs necessary.
As for the suit itself, it had better be a two-button, single-breasted jacket. If it's a 3-piece, leave the vest at home. Do NOT wear sneakers of any kind. I don't give a fuck who you are, you are NOT Justin Timberlake and you are NOT a Grammy Award-winning hip hop artist so you will NOT pull this look off, except in the eyes of people whose opinions on this matter are irrelevant. Wear black dress shoes, period.
Don't worry about appearing overdressed. It's always better to be overdressed than underdressed. Funerals are about showing respect to the deceased and that's it. Your way of showing respect is to make clear that you put a little extra time into your clothing for the funeral because the deceased deserves at least that much. This is not a time to avoid a suit because of the inconvenience of having to get it dry-cleaned or getting a new tie or because you're worried about how you'll appear to everyone else or you're worried about being overdressed.
The deceased is far more inconvenienced than you are at this point, so by wearing the suit and wearing it in a manner that shows you put a little effort into it, you are telling the deceased "hey, I cared enough about you to at least spend a few extra minutes getting dressed for your funeral." And screw what anyone else thinks if you appear overdressed.
You won't be better-dressed than the deceased and if you are, that's THEIR bad and not yours. If the deceased's own family shows up wearing motherfucking POLO SHIRTS, fuck them. Don't placate the lowest common denominator here. You know what's appropriate and classy and if they don't that doesn't mean you stoop to their level.
If it's really hot out, deal with it. The dead fucker in the casket is probably going somewhere a lot hotter than where you're at. And this is also why you should own two 100% Italian linen suits. One light-colored earth tone for outdoor weddings in the summer, and one in black or dark grey for funerals in the summer.