Trevor, I'm all in on your side
The 4.5 is a nice engine but there are almost ZERO options for doing anything to increase performance. I've been a member of the forum since ... 2005 I think, and I've only come across one or two threads that had any substance in the performance realm. I like how anything Chevy has parts availability ANYWHERE, and any independent mechanic worth his salt can work on one. Try that with a Mercedes engine - if you break down in the middle of Kansas somewhere, it's unlikely you'll find a mechanic who can get you back up and running.
The 2.8 is an anemic engine. It's nicely engineered and will last well after the zombie apocalypse ends, but if you have hopes of it providing zip and zing ... well, no.
You are unlikely to find a shop that has off the shelf mounts for what you want to do, but one avenue you might start out with is Kennedy Engineering. My dad used them to mate an aluminum V8 to a Porsche 914 transaxle (bell housing, adapter plate, more). If they have nothing to offer, ask them for leads or shops that might. And keep going. Much of what you're wanting to do will need specially fitted <noun name> requiring custom fab or custom installs. If you're up for it, here are some things to look for:
1. cross member is in the wrong place for a Chevy. Consider a custom oil pan (go to HotRod magazine and you'll find a TON of folks who do those for cheap) or a dry sump
2. Steering mechanicals may not work to your advantage. Consider a rack and pinion from a Chevy Lumina - I'm told they go in without a huge amount of hassle and will offer you more room in the engine bay
3. Control modules for the fuel injected engines are available aftermarket too. If you get a newer CSB, you'll need to figure out a fuel pump that'll work for your setup - the stock pump won't deliver the pressure or capacity necessary.
4. The drive shaft will require customization. Any reputable machine shop will be able to weld the rear Benz part to a front Chevy part, plus balance it, etc. I live near Washington DC so I have access to a ton of places here, Baltimore, etc
5. Suspension won't need major modification, as the Chevy and Mercedes engines aren't massively different in weight
6. You might have to modify the transmission tunnel and of course, make a tranny mount to fit it
For cost, assuming you have nothing at your disposal yet, an engine/tranny combo from a newer Chevy will run you $2k or more (probably more). Aftermarket controllers range from $250 to four times that. A custom oil pan will go $100 to $500. Driveshaft splicing $200 or so. Assuming you're doing all the welding, placement, fab, etc etc, look for another $500 in associated costs and bits. A Lumina rack may get you at $200 ...
The stock rear end, the radiator and the AC system can stay in place but again, mating Chevy bits to Mercedes bits will require customization.
Again, this is not an impossible job, but will require someone handy with tools and fabrication. At a guess, and this is a straight guess, having a shop do this all for you would run you well north of $10k ...