They're not exactly trouble free...they are still in the years for bad cats, bad valve bodies in the autoboxes, bad timing guides. The VQ is more expensive and more difficult to work on than the 2.5 (size, weight, cram factor). So if you find one that was looked after...you stand half a chance of having some fun without a lot of wallet pain.
From going through the whole car [05 SER], I would say to just get rid of the factory exhaust manifolds for some stainless headers.
Next up would be to listen for any ticking in the timing chain area as those guides had issues with the polymer breaking on the chain guides.
Last I would install an external transmission cooler, AND filter. The valve body is like a big multi layer sandwich, and when it gets hot its easily warped. The replacement valve body is around $1k so keeping the valve body cool is a must.
Better yet, just buy an SER with a manual 6 speed.