The 147 is a great car IMO, 147 has super light steering which is very precise and it also has the turning circle of a moon, no really, a moon. Out of the city it's great to drive and a healthy car will lap up the miles. The petrol engines sound good but by today's standards are a little thirsty. The diesels feel a little heavier at the nose but there's a ton of grunt to power out of the corners. Healthy suspension makes a big difference here. The driving position is excellent with very supportive seats but they need to be because the ride is a tad firm... I'm lying, it's very firm.
The interior is a nice place to be especially if you get a car with leather seats, Lusso trims and above tend to have leather. Also Alfa didn't use hysterically terrible plastics for the dash etc which is nice. The interior is quite spacious in the front but the back is cramped and there's really only space for 2, especially in the 3-door cars. Despite the typical rot-box/reliability issues you hear about Alfas, my 147 has only suffered minor niggles, battery/EGR for example. Keeping up to date with maintenance will help a great deal here.
The Twin Spark (1.6 or 2.0) engines are designed to use some oil so it's important to check and top up the level regularly. Aside from that it's the cambelts you need to check, Twin Spark belt replacements should be every 36k or 3 years whichever is sooner. The cam variator tends to need replacing every other timing belt change, a worn variator tends to sound like a clattery diesel and it's a cambelt off job so look out for this one. I've driven the 2.0 Twin Spark and it laps up the revs even if it lacks a bit of umph (that might just be me being used to the diesel) but the noise is nice. They average not much more than 35mpg with a combination of urban/city driving, maybe 40mpg if travalling on the motorway.
Diesels on the other hand don't normally use oil but need cambelt & waterpump replacements every 60k or 4 years whichever is sooner, £350 is about the going rate for this work. The usual EGR malodies are common and can be avoided by cleaning them every few thousand miles. I've had my 147 JTDm for almost 3 years now and remapped it has bundles of torque, power and even decent economy (45mpg+) although it does sound like a taxi when it's cold. Get one with the Q2 limited-slip diff for the best results.
Servicing is fair, I pay around £150-£200 all in for a yearly service depending on what filters I choose, Engine Oil, Motor Oil and Car Engine Oils from the largest UK independent online supplier of Performance Oils
will see you right for service kits. Rust shouldn't be a problem but do check the floorpans from underneath and the sills, which can be damaged and begin to rust due to using an incorrect jacking point. Check for any electrical gremlins and make sure everything works properly, but generally the 147 is reliable here.
The suspension has a number of areas to check, the front upper wishbones squeak like mad when worn but £90 per side should sort this out. Lower wishbones should last longer but they cost around £80 each and the subframe has to be removed to change them so give any car a good test drive have a good listen to the suspension. Anti-roll-bar bushings and droplinks can also wear and the former is hidden by the subframe but drop links and new ARB bushings are not expensive, just the labour. The rear suspension is made up of 4 radius arms, I don't know much about these but my 147 has done 77k on bumpy roads and I'm still on the original arms.
See the guide here for more bits to check for: Alfa Romeo 147 Buyer's Guide
There's some things I'm bound to have missed but you get the general idea, sorry I can't be more specific with the petrols, I need more practice. People call me a philistine for choosing a diesel but they're still good fun.
Best of luck with your search!