I want you to do some more shopping... and this time stay away from MM.
You can damn near get a set of EIGHT full custom S60 pistons from Wiseco for $900. A set of four custom pistons will run you around $600 with rings and pins. At $780 for rods I hope they're Pauters... because you can buy Pauter rods for that much.
I've never been a fan of expanding stroke anyway... especially when you consider that the gains found from the increased displacement hardly warrant the cost of the crankshaft itself, let alone the other parts or things like camshafts which are required to get a true gain out of your new girth.
This is why forced induction is usually the best option for engines with weak aftermarket support. I really really don't want to step on any toes here but I still haven't heard any rave reviews of peoples 2.1L engines. While no one is talking them down I think it may be because they're embarrased at the hp-gain/$$$ ratio they had at the end of the project.
I see where you're coming from - if I went for a turbo, i wouldn't bother messing with the stroke, wouldn't do too much with the bore either, would get some nice 86mm low CR pistons and go. i also just about agree that bang for buck turbo is the probably best bet if you stick with standard internals, but if you factor in a proper rebuild and do the turbo thing right, it will cost more.
having said that, quite contrary to the not hearing rave reviews of 2.1 conversions, I'm absolutely chuffed with mine. It's jsut what i was looking for, but then again i was always realistic about what I would gain and also knew exactly what I wanted. I wanted a rebuild for reliability and performance. A turbo wouldn't have helped me for reliability and would have affected my fuel economy pretty badly, which I didn't want as I do loads of motorway miles.
The car is way faster with the 2.1 in it, I've driven a 1.8 since and been surprised how slow I thought it felt. Where it was the same performance as my bro's M44 e36 before in a straight line, now in 3rd gr alone from low rpm I can pull out from behind him and overtake - that's a huge difference in performance. With the ITBs etc I'm really so happy with the car. Sure I've spent a lot of money, but its very fast, totally reliable and an absolute pleasure to drive, it will also last for a long long time, as its now basically a new engine, not a 15+ yr old one. In short, if I could make the decision again, I'd def do it this way again, as if I'd gone turbo i'd have spent more and probably been less happy with the finished result in terms of reliability, fuel consumption etc.