if i were you, i'd take propshaft and rear axle too. that's if the owner doesn't want an arm and a leg for it. in fact, i'd probably offer him 150$ for all
you'll need socket wrenches and ordinary wrenches sizes 10mm, 13mm, 17mm, 19mm, and an angle grinder with several cutting discs will be handy if there's electricity source nearby. couple crowbars, big hammer might come handy too
with condition of the car as it is now, test drive is obviously out of question, so you'll be basically be buying a cat in a bag. generally, you'd be much better off buying parts from a running car that you can test drive.
one danger i can see is that if it's been on it's roof for a very long time, oil has drained away and gears inside might have become corroded from condensation. you can test this by removing a cover plate from the rear of the diff. if gears inside are dry and rusty - walk away. condition of the diff will also be a good indication of what's inside gearbox.
as i've already said, 131 gearboxes are generally bulleproof. i work on 131s and 132s (same gearbox) daily, and 90% of them work perfectly fine even being 40yrs old.
failures i've seen over the years are:
noisy gearbox - result of corroded insides, cars that have not been driven in decades
faulty 2nd gear sinchro - cars that have been driven hard. off the traffic ligt full trottle up to redline, quick shift to second, you miss the clutch timing and viola! repairable, unlikely on a station wagon. it's probably been driven by a soccer mom.
cracked/chipped reverse gear - unlike 124 gearbox, in 131 for reverse to be engaged you don't need to pull up or press down on gear lever. for some reason people tend to shift into reverse before coming to a full stop. this results in cracked reverse gear. my wife actually managed to put our 131 into reverse once without using the clutch at all. naturally, she killed reverse gear instantly.
unlike 124 gearbox, bearings never fail, forks don't get worn all in all - much better han 124gearbox
oh, one more thing - check if it;s 5speed! there was a 4sp version of the same gearbox - looks identical on the outside
there are lots of parts on this 131 that you may also find usefull
rear trailing arms and panhard rod
front calipers and discs
whole engine and everything around it. distributor, starter, alternator, head, cams etc, everything will bolt right up to 124
, now that is dedication to the Fiat community 
