Like WhatsThisFor said, corrosion is the main thing to look for. These generally aren't undersealed as they don't spread salt in Japan. If you've a year or few to go, don't overthink this - get on ground with torch and look at the structural stuff.
MOT history will indicate whether the owner was on top of maintenance (did they wait until those advisories became majors?) and if rust has got really bad.
A 'sniff test' will check for head gasket issues quickly and cheaply. That's generally paying out £3500+ for a (second hand) replacement engine to be fitted.
Walk away from any blue smoke. White smoke on start-up may be water vapour, LPG valve saver being burnt off or... smoke.
Plugging in a code reader (select Nissan Consult II protocol, not OBD2) makes sense. The socket is behind the little coin tray by the driver's right knee.
Get on the ground and check the inside corner of the rear tyres for premature wear - tired rear springs means they'll wear very fast there and you'll get blowouts/have to replace tyres years early.
Always go for a test drive. Should be a quiet engine with no clunks or rattles. Go for cars where the owner talks about bushings etc being replaced... that's an indicator that they're a preventative maintenance kinda person who can afford to spend money on making it right.
It's much less risky buying these cars - no turbo, no direct injection, no having to plug into computers to change parts. Any time-served mechanic will love working on it - go to back-street/independent garages.
Much of the little bits of maintenance like droplinks can be done yourself. If doing work, essentials are a spare 10mm socket or 3 for when you drop it into the engine bay, magnetic pick up tool for fishing out nuts and bolts you've dropped into the engine bay, non-magnetic grabby-pick-up tool for fishing out non-metal parts you've dropped into the engine bay, ramps that don't catch the bumper when you drive up them (used to give you room to get underneath and remove the undertrays so you can retrieve the things you dropped that somehow made it right through the engine bay), cheap angle grinder with decent cutting disk for the bastard droplinks. Small blowtorch for other bastard seized bolts.
With the money saved you can buy tools to do the next job