I have had 3 110's & still have my 2002 TD5 110, my Wife had a 2004 D2 which we had for 10 years and sold it a couple of months back mainly due to similar reasons as yourself, it was becoming more maintenance intensive and overheating in summer, I also do all the work myself on the fleet, I had just had enough, 2 head gaskets over the years and I knew the 3rd one was somewhere around the corner.
I also have a 1999 build G55 as my daily drive and it shares the off road duties with my 110, I would have to say even though my G is the oldest in the fleet so you do expect some issue on a vehicle that is 17 years old, it is probably in the region of having 50% less issues than our LR's. I still do a lot of work on it but most of it is preventative maintenance which I think you really need to stay on top of if you want reliability. Things like keeping the grease up to the centre CV joints etc, it all takes time but is better than spending the money waiting for them to fail and basically need to do the same job. So yes I work on it quite a bit still but to date is has been a lot more reliable than our LRs ever were, being an older one they do not have the issues with window regulators etc so I have been spared this misery, it is also far more basic and has much less to go wrong compared to the post 2002 G's.
I was in a similar position as you of what to replace my Wifes D2 with, I love the LR4 but as I live in Dubai pretty well all vehicles with air suspension start having the air bags fail and deteriorate due to the heat and harsh climate, couple this with my previous experience with LRs I decided to give it a miss. Our next serious contender as my Wife loves driving the G was a 2010-2012 G55, but for similar money and a 5 year unlimited KM warranty we could get a new 2016 200 series LC.
The heart was with the late model G55 with no or little remaining warranty but in the end we went with the new shape 2016 LC which offered long warranty and long term reliability. We now have around 6000 km on it and my wife loves it, it is a fantastic touring vehicle, probably the best you can get, it effortlessly eats up the miles and is a pleasure to drive but it is boring. If you like a drivers car this will bore you, I am glad we have it in our fleet as for the long trips it comes into its own and is far better than my G, but to be honest I love the G as my daily drive and if I really get bored jumping into my 110 really brings you back down to earth.
If I was in your position and could only have one 4x4 I would probably push the boat out and get the best G55 you could afford in the 2008-2012 year range to minimise the maintenance issues, yes they are expensive but they also hold value unlike most other vehicles and to me if it is a long term keeper the initial purchase price is not to much of of issue as if it is looked after and you keep the maintenance up to it you should have many years of happy motoring with a fantastic driver experience.
Otherwise I would go for the LR4 or 200 series over an early 2002-2007 G.
As you do most of your own work the Gs are of a similar nature to your D2 to work on, much better build quality but you will need a few new tools as there are a lot of torx fitting and a few other specialist tools required but once you collect the tools you are good to go. Prices of most service related spares are comparative to the LR items if sourced from Bosch or other suppliers and not from your Merc dealer, but yes some of the items which you need to get genuine can come at a cost as is the case if you need to get genuine LR items from the dealer, but if you have time most spares can be sourced at a decent price.
Thanks for sharing your experience, include your mastery of the language..