I can give you a few pointers about the mkIV as I bought one 18 months ago.
The things I've encountered on mine are:
Dashboards can go faulty or go completely blank. £300+ from Ren-Oh or repaired from £50 upwards.
Suspension springs can break and also the original spring/shock set - up is a poor/dangerous design. If a front spring breaks at the bottom pigtail, the rest of it can drop over the shock and potentially shred the front tyre! I've replaced all mine with new Bilstein sport struts and H&R lowered springs which only lower it about 20 mm over the standard old ones. The car has lost its wallowing, somewhat bouncy ride and now feels more like a sports car and you can "feel" the road which I prefer with my driving style
Air con systems - most don't work and can be a pain unless you throw money at it. Various components can go plus the electrics have issues. It may also need resetting using Renault diagnostic system.
Electronics in general can be a pain, loads of know faults with electric windows, alarms, sunroofs etc.
Early diesels as you mentioned are worth avoiding like the plague. Later ones are better with more power.
Mine's a 2004 2.0 petrol
Grand Espace mkIV with an LPG conversion with 167k on the clock. Decent engine to be honest.
Gearbox can have issues (mine has)...notchy selection caused by either poor syncromesh, old gear
or gear selector rod problems.
Rear view mirror was set way too low for me lol, had to buy a new button and fix mine 6 inches higher as the darned thing was in the way of my vision and I was ducking down to see round left hand corners. Try as I might I couldn't get the original button off the glass! This is probably not an issue if you're of average height
Getting to the sidelights and indicator bulbs needs a small pair of hands. Prepare for scuffed wrists. I struggled with even the normal dipped bulb replacements.
Mirrors are all a little bit small for my liking and don't give a great view of what's behind. 15-20% bigger would've been better although not as aerodynamic in my opinion.
Offside rear seat belt clip is painful as you slide over it to get in/out of the car. It sticks right up in the way so should have been designed better. A silly little thing but one that did my head in..as well as other people's bums in
The turning circle isn't as tight as it could be and I find I have to move my hands around the wheel rather than just turning it with one hand as I did with my old Volvo estate (gosh I'm fussy lol).
Saying that, I love the old bus. But, I don't think I'd buy another French car though to be honest after the issues I've had with mine.
Hopefully some of that will help, although some of it is admittedly more personal.