What's a '95 C280 engine weigh & can I get it home in the back of a '95 E320 wagon?
 

If I pull one at the junkyard that is. I see that if I take the 3rd seat out of the wagon it just about makes a perfect space for the oil pan of the engine. Would it put any more weight on the rear axle than say 2 150lb guys in the back seat and 2 100lb kids in the 3rd seat?

You'll be fine. Just put down the 2nd row of seats and push it more towards the center of the car and you will be good to go. Just take it easy on the drive home. If your SLS is functional as well even more-so.

Agreed. The car is rated for 1000lbs capacity including cargo and people, fairly evenly distributed. The rear axle can take a bit more, provided the SLS is operating properly and the tires are inflated to the full-load pressure (inside fuel filler door), but any weight behind the axle should be balanced with at least as much ahead of the rear axle, as Psycho says as far forward as possible.

It is really a nasty piece
 

Any big engine is, especially used. Last place I would want to put an engine, hard to get in, more difficult to get out, oil leakage and other fluids are going to happen. Can't you beg or barrow a pickup or a vehicle with a small trailer to do the job. The motor cycle / atv and lawn mower trailers are just perfect for hauling an engine.

I agree, try to see if you can borrow someone's truck or rent a light duty truck for the day.

Good ideas but when I went back to the junkyard the engine had been picked over a good bit, the intake manifold busted, injector harness missing etc. so I passed on it.

it would be difficult to strap down in the back in the event of a panic stop you are done. I wouldn't do it, you may even buckle the floorpan in the back.

Spend the $30 on renting a pickup truck, and make sure the engine is on a pallet or a few dead tires at least and tie downs. One panic stop will ruin your day.