How much of an upgrade will those cameras be respectively in comparison to my current E-520?
As I said, you'll gain low-light/high ISO performance -- I'd say at least one stops worth, maybe two -- at the expense of AF speed with your current standard 4/3 lenses. So to me, this really isn't an upgrade, more of a "side-grade". Unless you plan to use only manual focus. Although, if that's the case, as an E-PL5 owner, I'd note that manual focus on the relatively low resolution LCD isn't easy (especially if there's a lot of ambient light, i.e. outdoors in direct sunlight). This is why I bought the VF-4. But once you add a VF-4 to the E-PL5, the package isn't a whole lot smaller or cheaper than an E-M10. (You may also need to upgrade the E-PL5's firmware to support the VF-4. I did, but I bought my camera a while ago.)
The weather sealing of the M5 appeals to me
I'd also note that the E-M5's weather sealing isn't that big a selling point unless you also use weather sealed lenses. The catch with this is that some weather sealed lenses cost upwards of $1,000. So if the E-M1 is over your budget...
Two relatively cheap weather sealed lenses are: Oly's 12-50mm (around $400 separately, or $900 with an E-M5) (and note that at f3.5-6.3 it's a relatively slow lens) and 60mm f2.8 (around $400).
Also note that you might want to go with Oly's more expensive ($200) m4/3 to 4/3 lens adapter since, unlike Pany's adapter ($150), Oly's is weather sealed.
The new, 25 mm. M4/3 Zuiko Prime looks good.
I agree. It will also give you even better low light performance. But I'm not aware of it being a kit option for any of the cameras on your short list. So, for example, when you add the cost of an E-M10 ($700) + 25mm f1.8 ($400) + Oly 4/3 lens adapter ($200) = $1,300 which is the cost of an E-M1 body only. To be fair, you'd still need the 4/3 lens adapter with the E-M1 body, tho.