I wouldn't bank on this being a cheap or easy project,and only tackle it if you know exactly what your taking on as the 2.5 transporter isn't as simple as you think. Trust me I've owned one, and fixed it.
The 2.5 engine in the T5 is fitted to the T5 and 2.5 Touareg, no other vehicle in the vag range has this engine fitted, its completely gear driven with no belts or chains whatsoever, bear this in mind.
The 2005 build year vehicles suffer from, cracked and scored cylinder heads at the injectors, cracked manifolds, turbo failure, and MUST be serviced using the correct Gold oil or worn camshafts and tappets will also rear their head.
Leaking injectors are also an issue.
The air leak your hearing will likely be a cracked manifold.
Drive shafts, and stub shafts are also a common failure and £400 to replace, pattern parts are available but are inferior to OEM and will fail
I have a complete 2.5 engine, minus a turbo and manifold, and a recon turbo in the garage, that I have kept for a rainy day, its a good one and if your interested in selling on the van for a profit I'd possibly be interested in taking it on as I have many parts here to repair it.