Hi Doog,
I had my IS200 Sport for 8 years and loved it. I always lusted after the IS250, looked at one in 2010 even, still a bit expensive then. This year was feeling a bit better financially and took the plunge. I found a lovely '56 IS250 SE-L with all the bits, (except the sunglasses holder :P) an low mileage for around 8k, and fell in love on the test drive.
It rides smoother and you have more of wafting along ride, altho sporty enuff, the old 1 was the sport so a little harder, but i do love the extra toys.
A beemer of this age I wouldn't touch and the mercs look dull and aged.
Try not to go for less than SE-L auto and Sat Nav, you won't regret it.
The '250 has been a little heavier on fuel than the '200, but that's to be expected, about 300 miles per tank, got around 360 with the '200.
Hope that helps.
James