considering buying an older model I have read gearbox problems an issue.
any advice on what to check for before purchase

Check the buyers guide in the wiki  .
Experience is the only genuine knowledge, but as time passes, I have forgotten more than I can remember
Volvo V70 P2 2006 2.4 Petrol 170bhp Estate SE
MG Midget Mk1 1962

Previous: L322 Range Rover TDV8 3.6 2008; L322 Range Rover TD6 3.0 2002; P38A Range Rover V8 1999

rear subframe bushes
coolant level sensor
auto box
thermostat
water pump
secondary fuel pump
egr ,filter and breather
high/low shift motor
front air spring
front ball joints
folding mirror
fuel burning heater
glow plugs
diesel injectors

To name but a few... basically you need to check and recheck everything on the motor, crawl underneath and inspect everything as well... take a friend to look at it with you who can stop you getting all starry eyed and buying the first you see...

Read the WIKI on here for tips on all the common problems....

Put a wanted add on here and get one that's been looked after by someone who's an enthusiast...

TAKE YOUR TIME.... it's a buyers market...

Oh and I know you probably won't but forget the diesel and buy an LPG petrol, faster, smells better, more economical, better gearbox.... (but I would say that...) Pete
I now have permission from the boss to use Y.O.L.O. Enjoy life
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2014 L405 Autobiography SDV8 4.4 Loire Blue Ebony interior
2011 L322 Vogue SE 4.4 TDV8 Baltic Blue. Sold
2012 L322 Autobiography 5.0 Supercharged Ipanema Sand Sold
2002 L322 Vogue 4.4 V8 Epson Green (Fatty Offroader) Sold
-Click for Project Fatty off roader-

Any car can have any fault. You already know about the gearbox and the wiki, so there's good advice there.

I think Haylands is onto something as well. I've never run LPG (or diesel) as petrol makes the most sense here, but I'd seriously consider it in the UK. And those BMW V8 seem to take to LPG the best, although any can be converted.