I can possibly lend some helping hand with the cosworth setup on this one, it's been a steep learning curve for me since I've been messing around with it!
The MT75 box and associated transmission is a very very old design now, developed in the mid-late 80's the technology is approaching 30 years old. There is nothing really wrong with it other than it is big, chunky and the gearbox does not like anything above 350bhp/lbft. That is what people claim when it's fitted to the escorts & sierras, fitted in a lighter car it will do more. It basically rips apart between 2nd/3rd usually.
It also means you have to mount the engine 'inline' and not transverse like traditional to the puma and Evo, which is why Ben is suggesting the Evo route. It's a smaller transmission meaning you won't have to cut an enormous tunnel into the car and stronger and more modern. It will feel like a more modern car. The MT75 box is like changing gear on a tractor, it feels very agricultural.
That said there are many options available to upgrade the MT75 to make it stronger and better. But it's usually in the thousands. You then have a rear diff to upgrade at some point.
I am yet to start pushing mine, I still need to fit the strengthening plate in the front diff and get a spare MT75 box built up with shot peened gears and perhaps super finished gears too, I'm told this will be enough. I'm just driving carefully malign sure I don't stress the transmission too much at the moment. Still having fun though!
Whatever you decide, it's either massive amounts of work, or one very big bill. Either way if you want something different it's the only way to go!