Let's say there's a set of buildings with differing numbers of units in each. I would like to describe them as two or more unit buildings. Writing it out with the number, would I write "2+-unit buildings," "2+ unit buildings," "2-unit+ buildings" or something else entirely?
Most of this is coming from my personal experience, as I've not seen any professor or style guide talk about this. That might be the main problem here. I'll evaluate the three options you've presented and why I would go with one and not the others.
However, if you want to quickly know the answer, the tldr is I'd personally go with "2+ unit buildings".
1. 2+-unit Buildings
I suspect this option stems from thinking that this is a compound word. I'm basing this off of the inclusion of the hyphen.
The first problem, which isn't that big of a deal, is that it looks very confusing and ugly.
The more major issue I have is that it's an attempt to be formal, but violates a lot of formal style guides. Most style guides that I've ever been exposed to say that numbers less than 10 (some like Chicago Style say less than 100) in text should be spelled out. I've also seen some not allow starting sentences or phrases with numbers.
I just personally don't like and have never seen this option. It's trying to be formal when the most formal thing to say would be something like "Each building contains two or more units." If you're going to break formalities, just break them.
2. 2+ Unit Buildings
I prefer this option because it's more casual, which (I'm assuming) is better suited for the situation. I've seen it in use, though I don't think any so called Leading Authority has said it's the right thing to do in your situation, or any situation for that matter. I've most often seen it used on websites, games, or posters either implied or outright shown. I believe the reason for this is because it's quicker to read and easy on the eyes (at least my eyes anyway).
It also maintains sense when rearranged. What I mean is that if you were to write "Each building contains 2+ units." it makes sense.
3. 2-unit+ Buildings
I dislike this option the most; the first reason is for a similar reason I don't like option 1: It looks awkward. Though honestly, this is a nitpick.
The main reason I dislike it is because it doesn't make much sense. The plus as I understand it is a stand in for the phrase "or more", and should be right next to the amount it modifies. If you take that away, you are saying that each building contains a 2-unit. Not sure what that is.
I'm not sure what you're doing this for. If it's for something formal, I wouldn't do it at all; I'd rewrite the phrase and/or spell everything out. If this is for a poster or ad or something, I'd pick the 2nd option for the reasons given above. Hope this helped.