Best to start with the 2.6 190 as your base car, the different slam panel and steering box location makes life easier. But if I was doing it the steering box would be replaced by a rack from a w210.
I've done a few engine swaps on 190's, you dont want to use the m104. They are very nose heavy with the iron block 6cyl fitted, and even the 3.6 is not that fast unless your going to modify it. The twin turbo versions need alot of work to only get the power of a v8.
The 5.6 m117 is a nice easy swap without cutting the car around, and will get you to 300hp if you use the ece version. Or fit standalone engine managment for 350-360hp. The m119 needs some metalwork though due to the width of the heads.
The jz supra engines are an awesome bit of kit, and have a massive aftermarket support for tunning parts unlike the merc engines. There is a guy in oz running a 400+hp 2jz, it's quite the street sleeper.
As Jjr has said, the m111 kompressor engine can make 400hp when running a decent turbo setup on the standard engine. Mosselmann used to make a cast turbo manifold for just that. Running the m111 would also give the car a nice weight balance.
Forget the dogleg getrag box from the cosworth 190 though, it's a bit stronger than the normal 5 speed but not very nice to use. It's mainly the massively over-complicated gear lever linkage that is the problem. The later 6 speed from the w203 is much nicer to use when fitted with a short shift, it is also stronger too. I run one on my w124 superturbo at over 557lbft without a problem.
For your project I would go with a 5.4 m113 and 6 speed manual, it's all alloy so doesn't weigh a huge amount and you have the option of running the kompressor setup if you want more power. Run the w210 steering rack and fabrication can be kept to a minimum