Get a Pre purchase inspection before any thought enters your head about buying any ML. or rather any mercedes, period.
Forget about servicing costs - you are embarking on an expensive undertaking. estimate $2K per year to cover your
put aside extra to cover for things that have possibly been neglected by P.O. or their mechanic
(pays to know details about history, not just take a stamp as being proof of service) and definitely budget for stuff like transmission-related issues, cam sensors, water pumps, etc. Some early v6's had balance shaft issues - not sure what the exact deal is with those, but you'll read about it on foreign cars more than you will on local ones- I have zero interest in the V6 engine. I'd prefer the common rail diesel or the v8. Either way, they are a PITA to work on. from spark plugs to fuel filters - nothing is simple.
Noone can say how often things need changing on any car - it depends entirely on how you drive, how the P.O. drove, what condition the car is in - way too many variables to even warrant broad generalizations or statements based on assumptions (which is dangerous). Some things however should be replaced at every service interval - and that includes fuel filters, despite them being a recommended 60K item - just wait till you see the crap inside one, then you'll go buy yourself the special pliers to remove the clips and a box full of spare filters. Brakes - wear out. you buy new rotors and pads when they wear out and flush your fluid. end of story. routine servicing that should be undertaken at least every year, not just a 20K interval.
If you are a lead foot and race at every set of lights, your brakes won't last 12 months. You are stopping a 2 ton-plus land barge after all.
Mind you, the transmission won't be long for this earth if you drive it like you stole it. (or if that's how your wife drives?)
At least you can avoid airmatic. if you want one with it, then you better put aside about 7K and know that you WILL have to replace all 4 struts, the pump and filter/valveset at some stage during your ownership. Mostly because noone properly services the valves or the compressor. Just like the idiots who complain about Range rover air suspension failures, airmatic has the same issues. i.e. lack of servicing, moisture ingress and components with a finite lifespan (i.e. airbags) much shorter than conventional spring/damper assemblies, and with a higher associated cost to maintain.
the ML is underrated. Styling is a personal thing - I've always referred to the 163 as the Tuscaloosa Turd, because it's so ugly and poor american build quality. the 164 I thought was a styling improvement, but not much, and the 166 is finally a 'less ugly' brother to them both.
There are a lot of mechanical differences over the production spans of each successive model. Too many to list. You need to go into one of these with a very clear knowledge of what you are getting yourself into.
Reliable - that is simply a question of maintenance - whether or not you or a trustworthy mechanic with detailed knowledge can look after it and if something electronic fails, can you afford to fix it (replace module/controller etc) and can you be without the vehicle for a couple of weeks (maybe more?) if something does happen to go horribly wrong... like the TCU or conductor plate, or ECU, or ignition or something else needing a computer and a bunch of codes from Stuttgart to get it mobile again.
All these modern cars with vin-specific coded modules and associated bullshit are simply an expensive exercise in ownership. I'm of the opinion that once it is out of factory warranty, it's not on the purchase horizon, unless the parts are cheap and readily available as spares. That's a difficult pill to swallow when your mind is already made up.... or 'someone else' has made your mind up for you.
The reality is, there are plenty of MB's (not just ML's) which fall under this same mushroom cloud of impending electronic doom. It's not to say they won't be reliable - for the most part, they are... but not all of them are faultless examples of the three pointed star, and some pragmatism is required when assaying a model for purchase.
get an independent mercedes-benz specialist workshop to do a pre-purchase inspection and it will be the cheapest part of owning any Mercedes.