I too have a 380 SL - installing a 5.0L engine does sound very interesting. I've already had my engine converted to dual roller chain but quite a bit more than $2800. If there was an option to dump the 3.8L and replace it with a 5.0L engine, I might have went that way instead.
Like you, I am curious about the difficulty in installing a 5.0L engine in a 380SL and especially one tuned to high compression specs.
As long as you don't have local restrictions on engine swaps or emissions compliance, the 5.0 liter swap isn't at all bad for pre 1986 US spec motor going into a pre 1986 US spec 380SL. After 1985, you have the conversion to KE Jetronic and that adds another layer of complexity that's really not worth the effort. You can mix and match a bit with using the K Jetronic parts on a 4.2L or 5.6L post 1986 motor, with a few caveats, if you really want to.
A second option with almost as much power and similar fuel consumption to 3.8L is the later 4.2L. the 3.8L intake parts all bolt right up to the engine, as will the SL's oil pan and oil filter pedestal.
If you can find a recently rebuilt (top end rebuilt, the bottom is nearly bullet proof and hardly every needs rebuilding) 4.2L as an alternative to the 5.0 liter mills, I would seriously take it into consideration. 4.2L is a stroked 3.8l, in the same way that the 5.6L is a stroked 5.0L. The 5.0 intake manifold and K Jetronic pieces will bolt to the 5.6, but I'm not confident if that is true between the 5.6 and 3.8. I'll have to check the bolt patterns and dimensions.
In your case, you could convert to a whole other engine (from the single chain 3.8L of the '82 and earlier to dual chanin) for the price of dual roller chain conversion and get more omph, assuming no regulations would make that illegal or impractical. Possibility two is to leave the 3.8L as single chain and just replace the chain every 50K miles, which is a lot cheaper.
If you really aren't happy with the 3.8L and you don't accept the 3.8L's torque and horsepower curves (torque, in my view, being the more important of the two for day to day driving), there are a few things you can do that are a lot cheaper than flipping motors.
If you don't plan to tool around at the 120 MPH top speed and are willing to accept a higher RPM at cruise and a top speed of around 105-110MPH, you can go with 2.72:1 and 3:06 rear-ends. An open diff is easier to find than a limited slip version to find (diesels have 3:06 rears pretty often, sedans often have the 2.72 and 2.85 rears), which will give you more off the line. It will also work great if you decide later to go with a 4.2L or 5.0L motor, at the expense of some fuel economy and an out-of-calibration speedometer (shops that service VDO speedos can adjust this).