Dean --
You asked several questions...which Ken, Bart, and others gave some good answers to...I'll give you a few brief thoughts -- but overall, these are GREAT, easy-to-use, sensitive, light, extremely accurate (ID-wise) machines...
By the way, I asked about ten jillion of these types of questions before I bought my machine -- I had the same questions as you and asked literally DOZENS of forum users about their machines. I finally bought -- and I am not one bit disappointed! I trust you will feel the same if you buy one. On to your questions:
1.) You seemed to imply hunting coins in trash is your focus -- so yes, I'd get the Pro first (with 5" DD coil), and then add the 11" DD later on (assuming you can't afford both at once). I have the 11" DD and it is a great coil, very good target separation, and covers a large area quickly. In a not-too-trashy site, it's GREAT. No doubt though that the smaller coil has better "trash see-through" ability.
2.) Yes, they can pick up coins in proximity to nails. All detectors struggle with this, and this is a more "advanced" type of recovery that you should not focus on at first, in my opinion. Such a complex nail/coin target can alter the VDI reading (the nail will "skew" the numbers in some cases), and you might also get a "choppier" high tone mixed in with iron grunting -- somewhat similar to "iron falsing." So, trying to dig this type of target is something you might want to focus more on as you get more used to your machine -- you will dig alot of rusty bent nails and such trying to find these nail/coin complex targets, and thus it takes patience. But, this machine is indeed as good as most any, (better than most actually -- perhaps one of the BEST) in terms of allowing you to eventually find the coins near/next to/under nails. You might concentrate though on the easier/more repeatable targets until you get used to the machine.
3.) As far as the "good for coins" question, and the depth question, yes, they ARE good for coin hunting, and yes they ARE surprisingly deep. Obviously, this is a higher-frequency machine tuned toward gold and other lower conductors, and higher frequency machines are not usually as capable of seeing deep. But I, and many of us, have found it to do SURPRISINGLY well as a coin hunter -- both in its ability to "hit hard" on coins, AND to see surprisingly deep. I personally have dug 7" wheat cents with this machine, and can hit 10" coins in my test garden, in disc. mode (obviously, soil type/mineralization and other things affect depth, so this is not a simple question). Anyway, as surprisingly deep as this machine is in disc mode, it's even DEEPER in all-metals mode, up to a couple of inches deeper on coin-sized targets. And a nice thing is, even though your digital readout switches from target ID numbers to ground phase readout when you switch from disc to all-metals mode, you STILL have the 0 to 100 "speedometer dial" at the top of the machine that is giving you target ID number ranges even in all-metals mode. This makes all-metals mode a legitimate way to coin hunt, especially in a non-trashy site where you won't get bombarded with a bunch of trash tones. Still, disc. mode in this machine is awesome, with the ability to set your "tone break" at whatever VDI number you wish -- thus giving targets below that range a low tone, and above that range, a modulated VCO audio tone that has alot of information contained in it (clues about target depth, size, etc.) -- once you get used to how to interpret the VCO audio -- i.e. the machine's "language."
This is a great, sensitive, light, powerful, fast, deep, very accurate machine that LOVES low conductors, and really seems to like round targets. Most of all, it is easy-to-use and FUN to swing. It's the most fun-swinging detector I've used, hands-down. It's just plain easy; you will start finding good things right off the bat, I can almost promise you. I hate to sound like I'm giving a Fisher sales pitch here, but these are just excellent machines in my opinion.
Hope this helps you make your decision!
Steve