i can't tell you how to spot the different power levels although you should be able to check using the vin number or maybe even reg number. insurance companies often have them listed differently so put the reg into something like go compare see if it brings up any details.
things to look out for are your suspension arms, to see if they are noisey. top ones squeek and bottom ones knock / rumble. top ones squeek even more in cold damp weather so keep an ear open for them.
radiators, last about 5 years at best. before you go too far and get it too hot to test run your finger down the inside edge of the rad to see if its soft or crumbly, if it is, it needs replacing. budget on having to replace the thermostat at the same time as rad, they go in pairs!
if it sounds like a diesel on start up for more than a few seconds then the veriator has had enough and needs replacing, its a cambelt job to do that, so bear it in mind.
cam belt itself should be done every 3yrs or 30k miles approx, if its not been done walk away or having it done straight away, it will snap on you - there are plenty of threads of peoples who have been caught out!
always always get a car with service history, an alfa that hasn't been looked after won't look after you.
lastly engine temp, should get up to somewhere close to the 90 mark on the gauge where the fans will begin to come on at 90. if it gets more than half way to the mark after 90 its going to over heat, if it doesn't get past the mark before 90 the thermostat is stuck open and its not getting warm enough. that means replace thermostat and rad will almost certain need doing too.
check when plugs were done, they're long life ones as standard but can be changed to 'normal'. if the belt was done see if the aux belt and tensioners were done at same time and water pump also. don't take anyones word for it, ask to see receipts.
sounds a bit daunting I know, esp for a first time buyer but its the same with any car and you want to get it right when its your hard earnt money.
1999 156 1.8TS Sport Pack 3
2001 166 3.0 V6 24V Super
2003 156 GTA 3.2 V6 24V
2005 156 1.9 JTDM-Jet Sport
2008 159 1.9 JTDM Lusso
1997 GTV 2.0TS Lusso
2001 147 1.6TS Lusso