Kestral, you do realize that the plane you should get is the one you like the best which you find the best deal on and which is best suited to YOUR mission. IOW, which one I'd buy is meaningless to you because I'm not you and my mission is not your mission.
I'm all for gathering opinions on which model might be worthwhile to avoid from a mx standpoint or nasty flight characteristics standpoint or something like that. But in those respects, all three of your choices are going to be in same ballpark. The Dekota will haul more, go slower and burn more gas doing it. It'll also cost more to overhaul due to the extra jugs. The Arrow will haul less than the Dekota, will go about the same speed on less fuel and the engine will cost less to OH. The Mooney will carry about what the Arrow does in a slightly narrower/tighter cabin and go a lot faster on the same fuel burn.
The gear systems on both the retracts are relatively trouble free and easy to maintain. Along with the cabin being tighter on the Mooney, everything else is tighter as well. Its got one of the tightest engine compartments I ever had to contort my fingers into. That's not a huge deal, its just something else to be aware of. Mooneys also have wet wings which need attention every 20 years or so. If the tanks haven't been completely stripped and resealed in 20 years, plan that job into your budget ($8k+/-).
Which one would I choose? None of them probably. I got to fly a Mooney quite a bit in my younger days and I really enjoyed the plane. But these days something like an older C180 is probably more suited to my mission. But like said, the only time you should give a lot of weight to what plane the jackass' on the internet tell you to buy is when the jackass' on the internet are going to be the ones paying for it and flying it.