Problem appear magnified on here, cos people only join when they have them and they're generally the same ones! But it gives a good idea about the different engines. If there was 1 to steer clear of for reliability, its the 1.8. They suffer from head gasket failure - and its not a case of if - its when. Also, if they go, there are often problems with liners of the cylinders and the engine is never the same again. If you get a good one, they'll return about 30 MPG - but you'd probably want to change head gaskets at the same time as timing belts. They are a bit under powered in the Freelander.
The 2.5 V6 isn't much better than the 1.8 for HGF and general overheating problems. They'll also only return about 23 MPG - so unless you specifically want V6 power and are prepared to pay at the pump - steer clear.
The diesels are usually by far the best way to go. More reliable and should give you touching 40 MPG. It then becomes a choice of Rover L Series (up to 2000) or BMW TD4 (later). The L Series is a bit less refined than the TD4, not quite as powerful - but is more reliable and should give a couple of extra MPG. There are very few problems reported with the L Series on LZ - although a couple of months back there was a 'spate' of belts letting go - happened to 2 different regulars. Other than that problems are rare. It may not be of interest to you, but there is also a lot of tuning options available for them. The TD4 did have a issue with crank shafts snapping - but I haven't seen a report of it and its unlikely to be a problem. Fuel pumps don't seam to last long in them and they do suffer often from sensors failing and can be a pain to diagnose and fix.
My one's an L Series and I love it. I think its a great engine. It does have trouble getting me away from the lights without embarrassment, but once moving, performance is fine. It's a '99 and hasn't needed anything doing to it other than servicing in the 4 years I've owned it. It has towed my SIII SSW home on a transporter trailer without any issues, and towed our boat laden with camping gear bikes etc and laden with 4 adult size (2 grown teenagers) over the mountains of NZ without any grief. I can't really recommend it enough
So, if you go for a petrol, try as hard as you can to ensure there are no overheating issues - current or historic. Make sure you take it on a good long test drive. Get it up to speed and temp and if you can, take it up a good hill, stop at the top and make sure no coolant/steam is visible. If you go for a diesel, I suppose standard diesel checks for smoke etc should be made - but my L Series will create James Bond style smoke screens if I give it wellie and delay gear changes - but its fine under normal conditions (little puff on start up) and will give touching 40 MPG on a run and 35 MPG around town.
The "biggie" when buying a Freelander is the transmission. It doesn't have a center diff like 'traditional' Landies. The setup could be considered Front Wheel Drive and the diff that's connected to the gearbox distributes to the front wheels like a normal diif - but it also has a direct 1:1 pinion out the back that drives a prop to the rear diff to distribute power to the rear wheels. If this were all, it would be the same as driving those traditional Landies with diff lock engaged - it would soon destroy the transmission. This 'doesn't' occur though because the prop shaft is split into 2 and in the middle is a Viscous Coupling Unit (VCU). The VCU allows the small differences in cornering to slip (thus preserving the transmission), but will transmit torque when the speed difference is larger - ie when front wheels slip. Its a great system because its basically an automatic diff lock and you can change surfaces wihout having to stop or what ever - the transmission just automatically adjusts. With the Traction Control (that's fitted to most Freelanders) is gives them great 4WD abilities - limited by lack of low-range and ground clearance. However...
This 4WD setup can cause catastrophic problems for the transmission. The VCUs can/do tighten with age/mileage. This results in wind up when cornering and will ultimately destroy the transmission. There are tests you can do - after driving the car - feel the VCU - it should be 'warm', if its 'hot' theres too much wind up and it should be replaced, if its 'cold' its slipping too much and you're basically running 2WD. You can also do, whats called, a '1 wheel up test' that gives and indication of how much resistance the VCU is giving - there's ids on YouTube and info on LZ. VCUs may last any length of time from 20K miles to 200K miles!
So the VCU can destroy the transmission - but having mismatched tyres or tyres at different pressures will do the same thing VERY quickly. Tyres from different makes, although they may have the same spec (eg 195/80R15) will have different rolling diameters and similarly, if a tyre in underinflated, it will have a different rolling diameter. This results in a difference in spped between front & back axels that is greater than the VCU will allow to slip. This causes wind up even if you're driving in a straight line. My transmission blew after 400 miles on an under inflated tyre. So its vitally important (I know now!) to have matched tyres with equal pressures.
When the transmssion 'blows', it is usually the pinion in the IRD (front diff) that goes. This means you can usually just remove the props and VCU. This leaves the car as a 2WD. Its perfectly fine running 2WD (lots of SUVs these days can be spec'ed as 2 or 4 wheel drive - but you obviously won't have 4WD. To get the car back to 4WD you'll probably need at recon VCU (from £200) and IRD (from £600). If you're looking at Freelanders and you see one that hasn't got props - do not believe the seller when they say it was removed to improve MPG - it doesn't, its because the transmissions blown.
As I say though, if the tyres and VCU are looked after, the 4WD system is very good and capable.
Apart from me allowing the Freelander to cook its transmission, our Freelander has been almost perfectly reliable in our 4 years of ownership. I think other than servicing, I've only had to replace the ABS modulator. It really has been boringly reliable and I wonder if it is a Landie at times
There are various other bits and pieces to check when buying a Freelander, but I don't think there are any other 'biggies'. Most other things can be sorted with a bit of effort (eg rain getting in at the back) or are pretty low-cost (eg tie rods bushes etc). I think I've written enough!
Good luck.