some good comments above, My 07 Gt has been undersealed by autolusso but prior to this there were no issues, not even on the oil cooler or unions, there are clearly some good ones and bad ones.
re the age and mileage,well with 5k to spend I would go straight down the middle and go for something 6-7 years old with sensible miles on the clock, FSH isn't that important as it only means its had oil ,I would prefer a stack of receipts for things like the clutch ,cambelt and water pump..
DPF and EGR delete with re-map is desirable making the car less troublesome and a lot more fun to drive. it does wear out clutches faster though so be aware of this either as a potential expense or a job that has been done already.
water pump MUST be done along with the belt as its the reason for most failures,not doing it as akin to washing your feet and then putting on dirty socks..pointless...re the gearbox,well its the diff that goes so replacing that with a quaife will sort that issue on the M32. £600 fitted or look for one with it already done...and it will handle a lot better afterwards.
The 5pot 2.4 is a nice sounding engine for a derv and probably the choice I would make in the 159, its heavier than the 1.9 of course but the car is already so heavy a few kilos more isn't the end of the world. they can also be easily mapped to 250+ which will feel about right in the 159.
so for me 2.4 TI 07-09 lots of evidence of work undertaken,enthusiast owned rather than a dealer and rust free. my 07 GT is going strong at 101k and the 159 is a higher quality machine in comparison,for me the handling is just a bit ''meh'' hence I'm a GT owner.
go in with your eyes open and your wallet shut until you have found one that fits the bill.
they are out there, my mate just recently missed out on a 159 SW TI which had had Bilstiens ,eibach arb's a quaife re-map with egr and dpf delete and a really good history with lots of evidence of work and sensible mileage,owned by an alfisti and it went for around 5k.
beautiful cars the 159 and Brera good luck with your search