you don't have to cut your radiator support, but it will make changing a gearbox alot easier if you squash your motor up very close to the firewall like most do.
Yes you can get a 4age engineered with turbo, as long as the mounts are done properly.
You only have to cut the gearbox tunnel if your gearbox does not fit, lol I know that sounds dumb but it depends on which engine conversion you do, which floorpan you have, which gearbox you choose, where you mount your motor.
as for the maddat mounts for a 4agte, check their site for one, they have most of their products listed I think.
www.maddat.com i think.
There is nothing wrong with putting any other manufacturers motor in, choose whats best for the car you want to achieve, its your car, don't listen to others.
180kw would be a fun ride, 180rwkw would be even better. Both engines have alot of potential in a 1200 as they put the power to weight through the roof, I'd imagine you'll be doing or getting close to 12's. But it really depends on alot of things you do with your conversion.