Used 5DM1 vs 50D

With those lenses I'd definitely go 5D.  The 28-135 is a very nice general purpose zoom on 5D as long as you're not too critical of its IQ.  Don't get me wrong - it's good, just not great like an L lens.  From an IQ perspective, I doubt it would be any better on 50D anyway, and the focal length range is much better suited to the 5D's full frame sensor. I use a 28-135mm on my 5D - not that often, because I don't often have the need for a general purpose zoom.  But when I do, it does the job fine.

The 100mm macro will probably be awesome on the 5D.  Never used it myself, but the 100L is on my radar.

I assume you're talking about shoe-mounted flash?  Another reason to go 5D - if you're using that anyway, there's no advantage to the 50D's built-in flash.

My perspective comes from upgrading from a 40D to a 5D.  It was a noticeable bump in IQ, particularly at high ISOs.  Once I got my 5D I have rarely used my 40D.  Never used the 50D myself, but I gather from reviews that the IQ was comparable to 40D - the upgrade was that it has 50% more pixels (15MP vs. the 40D's 10MP).  So based on that, I'd guess the IQ would still be noticeably better on a 5D compared to 50D.

Quick suggestion: if you can squeeze another $150 out of your budget, try the 40mm pancake lens. Really fun lens - great IQ, so small and discrete, and a great FL for walking around photography.

Cheers!