The 156 SW is concededly smaller that your BMW525 Touring, more like a 300 series Touring, both rear seat legroom and luggage space.
156 2,0 Manual boxes are reliable and long lasting. Clutches lasts as long as the drivers lets them, mine have done 178k km now and seems OK. Mine consumes 8.3 – 9.0 litres fuel per 100 km.
The 2,5 V6 six speed box have had some problems, bearings and synchros.
The 156s are now up to 10 years old, and have proved reliable, but needs to be serviced by knowing people. Some, 1 in 10 especially 2,0s, consumes a lot of engine oil, so check often, cam belts and pulleys need change every 3 years, max 80k km, suspensions bushes last 60-100k km depending on road surface and driving habits.
Erik

 

make sure they have had the regular cam belt changes.

 

The cam belt change intervals were changed from 80 kkm to 60 kkm in a lot of markets some time ago. Since this influences warranty/no warranty, check and double check the service history. A ruined TS head can be very expensive.

As for mileage, I can only agree with Erik: 16V TSs tend to hover around 8-9 liters per 100 km (25 - 30 mpg).

My 147 used about a liter of oil for every 3000 km, but I drove it pretty hard.

 

Yes, broken cam belts is an expencive repair involving an exchange head. Pistons seems not to be damaged but often big end shells take the punch with the result that one of them go shortly after. The right thing to do is changing the shells as part of the repair to the head.
However, it was much more an issue on the early production 16V engines, pulleys have become much better and, very importantly, belt change intervals have gone down from 120k km to 60/80k km, max 3 years.
So it's not that much of an issue now, but always check documentations for last change, and in the teast in dought, have belts and pulleys done right away.
Erik
'99 156 2,0 TS, 178k km // '70 Spider 1750
Ex '96 155 2,0 16v S // '93 33 1,5IE, '86 75 2,0L // '84 Giulietta 2,0 L // '76 Sud 5M

 

Why not look for a more fresh JTS one?
It's a smoother engine than the TS, but has a better fuel economy.
I still have a 2.5 six speed manual, it's a workhorse for daily driving and a pretty fast car too...

 

I moved to the UK nearly a year and half ago, left behind my duetto and picked up a 1.9JTD SW to help ease the pain (searching now for a 101/750 spider)

I have two forward facing baby seats fitted in the back of sooty. There is more than enough leg room for everybody with front seats back (i'm 6,3). It is narrow though (think passat, golf, 3 series) - I'm told that sitting between the two child seats, performing emergency feed and calm the kids whilst travelling mode... is doable, but not that comfy.

Shed loads of room in the back for a double buggy (phil&teds) and any other travelling items (cartons of wine from normandy, few clothes and nappys).
We did a lap of nurburg ring loaded as above nicely

... test fit in a 2.0L

.. bought a derv, can't go past the torque and economy


get Diesel (did aust. get the derv?).. 600MILES off a tank, quick, smooth and pulls like a train all the way up to 120Mph.