The T25 turbo was fitted to the Rover 2.0 Turbo's such as the 800 Vitesse, 220 Tomcat etc, so there are loads about.
If it was me I would go with a T28, as it is capable of flowing more air, which will help power at the top end. The T28 was fitted to the 20V Turbo coupe, Nissan 200SX (And Pulsar), plus probably a few others.
I would worry about the TS gearbox going bang with the extra torque though. The Fiat Coupe Turbo gearboxes are far more robust, (And a lot heavier) than the TS item.
As for intercoolers, you can buy generic ones in various sizes. Just find one that fits and is as big as possible them plumb it in. You might find the intercooler and other pipework from a JTD can be made to fit straight in. The only thing is that Diesels don't have dump valves, so you will need to modify bits to get one to fit.
If you don't you will get randy pidgeon noises as the compressor stalls.
Giulietta 2.0JTDm (170), BMW 335i Cabrio