Considering a 2015 XJ 5.0 SC 47K mi (Currently have a lovely '07 XJR)
Plan to go test drive tomorrow

- Is 2015 a reasonably reliable year?
- Anything to particularly look out for?
- Worth moving to a newer model? (I prefer short wheelbase; seems hard to find in quick online search. Would like adaptive cruise and v8)
- Typically do all my own repair and maintenance

(Thanks for any advice)

-I would say the newer the better, so yes, the 2015 should be a much better choice than the 2010-2012 problem-prone models. You also get the more-tunable engine management system, the 8speed gearbox, a better touchscreen, and slightly better suspension calibration, if I'm not mistaken
-Besides the timing tensioner stuff in the aforementioned earlier years, make sure the water pump isn't leaking. Some of the cooling system hoses and pipes can be problematic, but that's been my experience on most cars anyway.
-Its definitely worth moving to the X351. I had my X350 for a very long time before moving up, and I would say its a better quality car overall - especially the supercharged 5.0 engine. I have the SWB, which I also prefer on this model over the long (it was the opposite for me with the X350)
-I also do 99% of my repairs and maintenance, and to be honest, its really not all that different from the X350. A lot of the components are either very similar or shared, especially the suspension control arm setup.

No regrets with trading up to the X351, even if I do sometimes miss the classic XJ shape. But I drove that classy, elegant style for so many years ranging from the old XJ6 to the X350, that I wanted to go in the opposite direction for the X351. So I made sure to make it as sporty and mean looking as possible without going to crass extremes. Totally different experience.

PS: Oh! See if you can find the elusive Flex Fuel model, which I stumbled upon without realising. E85 will give you a little boost in power.

I own a normally-aspirated X351, so no help there.
But your prospective 2015 purchase will have 3G telemetrics, which will be discontinued nationwide (if it isn't gone in your area already). Some of the features/benefits of Jaguar's over-the-air system you may find beneficial or not.