I've never shot the 22/45, but have shot a
bunch of MKIIs over the years. Same basic functions and internals, so I'll offer my thoughts.
I LOVE THE MKIIs!!!
I think they shoot well, are priced fairly, and will last for many years... If you're a tinkerer, there are lots of aftermarket goodies available, too.
The 22/45 differs primarily in grip shape and mag release functions. Grip shape is personal, but remember- it's a lot harder to change the grip shape on a 22/45 than it is to change grips on an MKII...
I'm not real familiar with 22/45 pricing, as I have chosen instead to focus on the MKII pistols. The 22/45 is usually a little cheaper than the "same" model in the MKII. Synthetic frame vs. steel frame is the usual explanation.
That said, I think $280 is plenty, especially if it's blued. Check around gunbroker.com and others to compare similar models.
Also, consider heading on over to
MarkII.org. Lots of good info here on Graybeard, but the MKII site is naturally more specific.
Good luck!!!