Buying used equipment is just like buying used car, you need to make sure you don't buy someone else's lemon. You don't go with a brand, you go with what the dealer has on hand, and condition. Our used dishwasher with repair bills and aggravation is just about the price of a new one. But no way we can afford a $10,000 dual temp pastry case, so we bought one on eBay for $2,000 and we have 3 trouble free years with it.
As far as equipment recommendation. If you are dead serious about the quality of your espresso I suggest you get a different espresso grinder. A well built and well maintained grinder can make a world of difference to your espresso quality. I have a lot of experience with MDX, and unfortunately, I have came to know it's flaw all too well. If you are on a tight budget, I would suggest a used Mazzer Major. As long as it is not abused, all you have to do is change the burrs and you are good to go. You can find them on eBay going for about $300 to $500 - less than a new MDX, and a better grinder. Mazzer Major is what I have at my shop in Hartford. If you want a really really good grinder be prepare to dig deeper into your bank account. Another forum participant, our very own Anne has a Anfim Super Caimano. It is a great grinder, but is it expensive.
Regarding moving the sink in between espresso machine and the blender station. It will most likely be used for rinsing blender bowls and milk pitchers. I won't designated it as a hand sink, but a work sink. I'd also buy a deeper sink so water won't splatter every where. And I'd keep a small hand sink near the half height swinging door.
Speaking of the half height swinging door, you might as will just not have one because soon or later someone will be on the wrong side of the swing.
Regarding water. You can contact cirqua and have your water tested for free. They are very knowledgeable, and have great system. Of course they want to sell you their system, but in my case the people at the company was honest to a fault. They told me I don't need anything other than a simple sediment filter. I was lucky, but Florida, at least Boca, has terrible water.