What do they usually need at this age?
Cost of parts and where to buy?
Are they a pain or a pleasure to work on?
Any differences between earlier ones and later ones other than headlights?
Do they offer sporty handling like the average bimmers?
Is the engine wiring straightforward?
What would be the biggest model specific hurdles to tossing an ls1 in one? 
The very late ones had the engines from the upcoming newer E-Class (W210?). I've had a 300E with the older lump and a very late (like last year of production) E280 with the newer lump. Liked the newer one much better. Also, like with all Mercedes, the later the car the better it is. Early ones can have issues with seats collapsing and didn't have the solid feeling you get from, say, a late (spot a trend here?) W123. Later ones are much better.
No sporty handling, they're cruisers built for long-distance comfort unless you're talking 500E/E500. That one is a hot rod in the best "German engineers have a sense of humour after all" sense, but they're holding their value rather well and not exactly cheap cars to run. While I like the idea of one, I'd probably get an E34 M5 instead.
I found the ones I had pretty easy to work on. They're very well engineered and hold together extremely well when looked after.
Forgot to mention - for a clean high-speed long distance cruiser with no sporting pretentions, keep an eye out for a 400E/E400. It's not as nuts or sporty as the 500, but they're much much cheaper and one of those cars that just seem to be built for a quick 1000 mile trip.
The 400e w124 is a great buy right now. They can be made into a more sporting machine pretty easily. They came with a lazy 2.24 gearing for economy and actually get decent mileage. Any of the W124 cars can handle much better with the addition of bigger sway bars/shocks/springs. Buy the best maintained car you come across and don't get suckered in by a car with no records. The previous owners care plays a huge role in these cars as they are more maintenance intensive than a civic.
ccrelan wrote: Buy the best maintained car you come across and don't get suckered in by a car with no records. The previous owners care plays a huge role in these cars as they are more maintenance intensive than a civic.
Quoted for troof!!
My experience/knowledge is with the 300D:
"What do they usually need at this age?"
Head gasket and timing chain. Maybe glow plugs ($20 each, not terribly hard to change)
"Cost of parts and where to buy?"
Engine stuff is ex-freaking-spensive! I'm talking internal engine stuff. My oil pump, chain, pan gasket and oil was $500!!!
But I went aftermarket (peachparts.com) and did a head gasket, turbo seals (they were fine though) and valve stem seals for $200 or so. Not bad.
"Are they a pain or a pleasure to work on?"
I hate mine! EVERYTHING I do just blossoms into a nightmare. But, if you have a shop manual just follow the procedures and it'll be much easier. Nothing comes apart like you'd expect so I spent a lot of time figuring things out, that sucks. On that note, an oil pan is an engine out job on a diesel.
"Is the engine wiring straightforward?"
I think you want a 93 or later to avoid the bio-degradable wiring harnesses. Or budget $1500 for a new harness 
"What would be the biggest model specific hurdles to tossing an ls1 in one?"
I can't speak to this specifically, but I think the width might be an issue on the W124's?? But, it's such a solid chassis, the added power and torque would not be an issue.
Arent 1993 and newer the ones with wiring problems? I am almost certain the w126 cars missed that year range, so that would be at least 1992 and up.
New issue of Hemmings S&E has a buyers guide on W124 500E/E500.
Travis_K wrote: Arent 1993 and newer the ones with wiring problems? I am almost certain the w126 cars missed that year range, so that would be at least 1992 and up.
I'm not sure, but I thought it was the 1992 and earlier, and even 1992 was hit and miss. Just do a google search and see what's out there.
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