I think 50D comes closest to 5DII, handles pretty similarly in a lot of ways, image properties from both have the same overall "feel" (so see pretty similar post processing needs), just the 50D's 15MP are a bit smaller and has a tiny bit less fine detail. I have mistaken some of my 50D shots for 5DII shots, until I looked in the EXIF.
AF is faster and more capable in some respects on 50D, better at tracking moving targets, for example. But still laid out and controlled in the same way. Both cameras have 9 AF points. Both cameras have the option of some focus screens, such as Precision Focus and Grid types, though they aren't interchangeable between cameras.
5DII has expansion points, which 50D doesn't... But 50D has all cross-type AF sensors, which 5DII doesn't.
Size and shape of the cameras are about the same. 5DII is heavier and lacks the built in flash of course.
Slight differences in control layout, such as the button to light up the top LCD swaps ends on the row of buttons and the Erase/Image Review buttons are swapped... On one the row of buttons are to the left of the LCD and on the other they are below the LCD. But it's really not a major problem switching back and forth between them, they are obviously from about the same generation and group of designers...
Especially major things like Main Dial, Quick Dial (on the back) and AF controls are all in the same places and operate the same way. In the menus many of the features are the same or very similar: Micro Focus Adjust, Quick Menu access, My Menu, etc.
50D doesn't have video, though, so it also has none of the menu items related to that. Both have self-cleaning sensor. Both have Live View. 50D has the same "Creative Modes" on the Mode dial, plus C1 and C2... plus what Canon calls the "Basic" modes, the highly automated icons such as "running man", "macro tulip", "mountain view" (or is that a volcano?). The 5DII has C1, 2 and 3, and lacks those "Basic" modes.
50D uses the older tech BP511A batteries and chargers, not quite as good as the LP-E6 used by 5DII, you might need an extra one or two but generics/clones are widely available and considerably cheaper.
Both use and can share Compact Flash. Both like and can benefit from UDMA and perhaps 45MB/s or 300X speed memory cards.
I see 50D refurbs at Canon's online store for about $700. Seems quite a good deal to me.