So I was looking at the Bugera
Did you know an anagram of bugera is garbage? Good thing you already ruled this out on your own.
So ideally adding an 8 ohm 4 x 10 be perfect.
Nope. This is frankly a horrible plan. The power gets all lopsided between each speaker, and you end up effectively having a 2x10 x2, meaning the 4x10 is probably only getting about half the power it's rated for (why people ever mention all the stuff they have and the pertinent specs when they ask these kinds of questions?). The only time the mismatched impedance thing works is when you have half the cab and twice the impedance, and the amp can handle whatever the resulting load is (usually 2.67, unless you find a 16 ohm cab to go with an 8 ohm one, then it's 5.33). And you'd want them to be as closely matched as possible (ie the same brand and series of cab).
A PF500 is a solid if unspectacular head. I don't think that's necessarily where your problem is. And at any rate, $300 isnt going to get you where you want unless you seriously luck out on an insane deal (best not to count on that). And even a 1000w head is only going to get you 3db, and then you have to have enough cab to use it (which in the real world means about 1500-2000w worth of cab).
Meanwhile, you have 3 cabs that you can't use together, and at least one of them is completely superfluous aside from being able to use and leave at a different location than the other identical one. You COULD sell the 2x10 and add the proceeds to the $300 budget and search for a 2 ohm head, but that might not yield much more power, and you'll be stuck carrying around 200 lbs of cab to every gig where you need that much firepower, plus losing the option of a smaller cab.
So what I would do, is sell both of your 4x10s (and maybe the 2x10) and use the proceeds plus the current budget towards a bigger and/or lighter cab or pair of cabs. For instance, a Genz Benz Uber 4x10 will be 4 ohms, but rated at 1000w and they only weigh about 70 pounds (20 or 30 pounds lighter than your peaveys I bet). Those sell used in the $500 or so range. You'll greatly upgrade your tone, the cab should take pretty much whatever the Ampeg can give it, and you'll be saving weight.
If you have anything left after that, you could think about either saving for a while or selling the 2x10 and/or PF500 for a higher wattage head then. $300 is tough to find stuff, but $400-600 and up and you start having options for 600-1000w heads. An Ampeg SVT 7 Pro is 1000w and they go used for about $500 and are pretty common. That and an Uber 4x10 should get you far. If you did get a 2 ohm head, you could also think about 2 Uber 4x10s. If you needed any more volume than 2 of those could do, you'd need to save up extra for some range muffs to protect your ears.