That's the problem with good deals, they can be very difficult to ignore. I have several cameras that I acquired because they were good deals including some Hasselblads, I have no regrets of buying them, but I also have more cameras now than I know what to do with.
If the money is easy, buy it and then figure out whether you like it enough to replace the 503, you can probably sell it off (or the 503) and come out at least even. If the money isn't easy, pass on it, and use a bit of the money for more film or whatever. More good deals will come your way, that's been my experience, anyway.