Quite the fleet you'll have if you buy a 147, and from three completely different generations.
So far my 147 has proven itself to be dependable and always starts first time. The suspension is shared with the 156 and GT so it's double front wishbones and radius arms at the back. A creak/squeak from the front usually indicates worn upper wishbones, budget £100 per side for this repair. Less common but more likely on higher mileage cars a front-end creak can be the ARB or it's bushes. I'm not sure of the possibility of just changing the bushes but the ARB is around £100, more if you have the Q2 model. There are four rear arms, two short and two long and these cost around £35 for the shorter arm and £50 for the longer arm. I'm not sure what worn arms sound like but any noises from the back could be the arms or the drop-links. Worn front suspension can cause inner edge tyre wear, but so can poor alignment so it's worth checking both.
I've never found a definitive answer to the DPF question but I know that the 147 had CF3 and CF4 diesel engines, EGR problems are common but is often a simple fix. It's fixed onto the inlet via 4 long bolts and can be dismantled for cleaning quite easily, replacements are around £120. The problem occurs when the EGR solenoid becomes stuck open and you'll find the car has NO power low down, a similar symptom to a faulty MAF sensor or air leak.
I have the 1.9 150bhp JTDm 6-speed manual with the limited slip-diff which I would highly recommend, the gear change is smooth and from my experience problem free. The only issue being the gear linkage bushes which wear over time and cause a stiff/sticky gear change, but this is a fairly easy job and only a few quid for the parts.
When you're viewing any cars I'd make sure you test drive it and check the clutch. A worn clutch will slip in higher gears and set you back a few quid, and anything around £900 if it needs a dual mass flywheel replacement too. Timing belts should be replaced at 60,000 miles or every 4 years which ever is sooner, the water pump should also be replaced at the same time. £350 seems to be a good price at the moment for this repair. The experts say that the water pump tends to seize up first and cause the belt to fail which is why the change interval was reduced from 72k.
Check the buyer's guide from Alfaworkshop too.
Alfa Romeo 147 Buyer's Guide
Best of luck with your purchase.
P.S. If you're interested I still have some recent-ish Parkers valuation packs for some of the 147 trims.
147 Sport Q2: Autolusso Remap + Swirl Flap & EGR Delete, Mocal Oil Cooler Kit, JTD Performance FMIC Kit + High Flow Elbow, Wizard Downpipe, PF Pads + 5.1 Fluid, Autolusso Braided Brake Lines, MTech Short-Shifter, Autolusso Silicone Turbo-to-MAF Hose, Refurbed Q2's + Some to add later.