How do you have a situation where you only own a car for 2 years and the power locks "always" go out?  One failure of the locks with less than 150,000 miles on the vehicle is too many.  How many failures did it take for it to become "always"?  2? 3? 5?  And what other parts were there that were "expensive" from the dealer?
At only 110,000 miles, a vehicle should have zero failures for at least another 40,000 miles.
As for selling it for what you paid for it, did you really?  How much did you pay for parts and maintenance over the period you owned it?  You get any of that money back?  Or are you, like most people, ignoring a couple thousand dollars in repairs for things that should never break (like locks)?  I could sell my 951 for it.  But that would be ignoring nearly $20,000 in parts and repairs over the seven years I've owned it.  
I love the car and I think it's actually been pretty reliable for a 1986 model with 170,000 miles of pretty hard life on it, but I'd be a damn fool to suggest that it would be a great vehicle for someone whose only interest was that it "caught their eye."  If you've always wanted one, it can be worth it.  If you just saw one and thought, "hey, that's cheap to buy, cool" then you really should not get involved.  
ZV
		
		
	 
The power door locks on the ML were a bit unusual. They would tend to stick. When one door lock stuck (would not go either up or down) it caused a problem with all the door locks. The body computer kept trying to either lock or unlock all the door locks to correct the faulty door lock actuator. The actuators were about $250 each from Mercedes, but could be repaired by doing some disassembly and lubrication. On my particular MB, I had 3 of the 4 door locks require maintenance. That is what I meant by my statement that the power locks always go out. If you check MB web sites, you will also see this is a very common problem for the ML series in this year range. 
Zero failures at 110,000 miles on a 2000 model vehicle? What world do you live in? These vehicles are loaded with bells and whistles. The more bells and whistles the higher the probability of failure, especially on a 2000 model vehicle. 
All cars require parts and maintenance during ownership. Whether it is a battery, oil and filter, brakes or wiper blades. Some items are normal maintenance and others are extended repairs such as axles, water pump, or power door locks. Fortunately, on my ML, excluding normal maintenance, I was able to sell my vehicle for what I paid for it without any extended repairs. These vehicles are designed to and can go into the 200 or 300 thousand, or more, mile range without major repairs. 
I often compared my ML430 to a high performance tank. Mine originally sold for around $50,000 dollars. It has great creature features, 4 wheel drive, 5 speed transmission, was Motor Trends 1998 Truck of the year beating out the Volvo and Cherokee, 8.4 inches of ground clearance for its 4 wheel drive, and handles like a sports car. Its 5000 pound towing capacity is great for about anything. If you find a fully loaded ML with the Bose sound system with 6 disc CD changer in the rear, built in Digital StarTAC phone, Power Skyview Top, or third row seat, you found a nice one.  But the best feature is that it is a Mercedes Benz. A very special status driving this car, and for how much? Normally you can find these cars for less than $7000. This is about the best bang for the buck you can get.
Even better if you can find an ML550 or ML63, or perhaps an AMG version. These are more rare and obviously more expensive, but can be found with some looking. I had to drive 300 miles to get the exact one I wanted after about a month of searching. 
The bottom line is: Yes these are fine automobiles and people who own them normally take care of them, meaning you can often find a well taken care of vehicle. It’s a special elite club of ownership, that often is important to your job or status. After I got mine, my wife kept wanting to drive it and we discussed even getting a second MB. This is a cheap way to become a luxury vehicle owner. 
Yes, parts are higher, and there might be some repairs that you will be forced to send to your MB dealer, but overall I felt it was truly worth it. I knew what I was getting when I bought mine, and now, so you do you.