Hi mate, welcome to the forum!
Firstly, there are a load of related posts throughout the forum regarding what to look for etc. I suspect one or two seasoned regulars would point you towards doing a thread search
However, from my perspective, you may be along the right lines by plumping for a JTDm if you are doing that kind of mileage daily. MPG-wise, you'll be looking at roughly 30 around town and late-40's on a run. Certainly not what Alfa claim, but then again, it IS an Alfa!
The JTDm engine is very rugged in general, and correctly serviced will run to very high mileages. There are a few things to be aware of, as with any car, that will need checking before you buy:
1. Cambelt and Water Pump. Ensure the cambelt has been changed according to the recommended interval, it certainly should have been changed on a 99k mile example by now. If not, it needs doing pronto! In the case of the water pump, it is a good idea to get this changed every 3-4 years or so, sooner if you do very high mileages per year. It is known to fail prematurely on the JTDm engine and again at 99k should have been done by now. Very important to get these done as soon as possible if no history/record of it.
2.Steering Rack. This is another potential weak spot on the 159, need to check the steering is spot on, so any strange noises turning the wheel/grinding on full lock etc. could point to the rack on its way out. Be mindful that some issues with the steering can also be due to the incorrect fluid in the reservoir. A much cheaper and easier fix if the case!
3.Diesel Particulate Filter. The perennial bugbear of most modern diesels, this is a filter in the exhaust that traps soot etc and burns it off by injecting fuel directly into the exhaust. You'll get messages flash up on the dash when it needs to regenerate, and usually requires a damned good high speed blast for 15 miles or so to clear it. Will clog up much quicker if the car is used for a lot of short journeys as it relies on the exhaust being up to high temperatures to regenerate. If it gets clogged up and isn't able to regenerate, you'll get poor performance until the car goes into limp home mode.
4.EGR Valve. You should know pretty much whether this is functioning properly or not, if it starts sticking or failing completely there will be a noticeable power drop low down, and the engine will feel very sluggish until over 3k rpm. Fairly commonplace on modern diesels, not just the 159.
That covers most of the major issues with the 159, a few things to watch out for, but nothing major like some of the German brands I could mention! I'm sure a few experienced 159 owners would be able to clarify some points for you, but overall I would say a well-maintained example is definately worth it. 159's go for thousands less than their German counterparts, but from what I know, it is every bit as well-screwed together.