I think it's pretty clear that in the original timeline, the McFly's certainly weren't dirt poor. They were living in a suburban neighbourhood after all, not a trailer park or cramped inner-city tenement building apartment. One does have the impression however that they are struggling at least a little bit; the furniture looks a little old, the decor is a little tacky (though that's more of a taste thing), and there's a lot more clutter. Contrast that to the epilogue where the furniture all looks like it was purchased less than a decade prior, the colour coordination is a bit more on-point and the place is just generally tidier (plus they now have a piano). Indeed I sort of see the house interior and decor of more a reflection of Lorraine and George's state of mind and self esteem.
Now I'm no expert on car makes (let alone American car makes) but I'm pretty sure George's car (the one Biff totalled) was a '76 Chevry Nova. Granted it was nowhere near as nice at the time as a 7-series Beamer, but from what I gather while an ageing model, it wasn't exactly a POS rust-bucket in and of itself. Indeed for a family car, it seems a little on the impractical side. Maybe they used to have sometime more sensible when the kids were little, but he traded for a second-hand Nova once they got older and Dave left school? As for the Beamer; as I said before, I can easily see that being just a company car.
I think everything we see the McFly's attain in the epilogue can easily be accounted for with George simply getting promoted to supervisor back when he originally deserved to. Maybe even another promotion to regional supervisor or something a few years later. Odds are Biff only got to be supervisor by taking credit for George's work to begin with, and even his advancement was depending on George doing the work of two people. So it would stand to reason that an unfettered (and self-confident) George would do so much better, and likely be recognised for his work.
Indeed just without Biff leaching off the family, upping their insurance premiums, and (most likely) borrowing money he never repays, making them late with the mortgage and bank-loans, I can see them easily going from "struggling" to "doing OK". With the pressure off, maybe Dave stayed in school longer, not having to drop out early and go work flipping burgers to help support the household. Instead he got himself an education, got an office job down the line . . . maybe even at George's office, since as middle-management he can swing a little nepotism his son's way to get him started.
What exactly was George's job? He seems to be some sort of middle class 9to5 corporate drone who works in a office full of other drones.
Some non-specific white collar desk job. We do see him with a printer-calculator at the dinner table with a bunch of papers (presumably to do Biff's reports for him) so whatever it is, it probably involves accounts or finance to some degree. Maybe an accountancy, maybe insurance. Doesn't really matter either way I suppose.