Frank, I have an HP-B. and to put it plainly, I love it! I had an Aztek that I used for general coverage but my Iwata has spoiled me to the point I didn't use the Aztec anymore and sold it to a friend. I'm planning on getting another Iwata for general use. To answer your questions:
1) No, there is no cap. But in a little over 3 years I've never had a spill.
2) It's excellent for freehand camo! The only one better is probably the Iwata Custom Micron series, which goes for $300.00 plus.
3) I only use acrylics, but I have heard others say as long as you thin them enough enamels are fine.
4) Very easy to clean, even if I break it down it doesn't take long.
5) The Iwata "plus" line features include, a dual-purpose cutaway and preset handle; teflon needle packing for solvent-based paints; larger nozzle threads for more secure fit and better centering of nozzle; redesigned tapered gravity feed cups for easy clean up and efficient paint flow; and single-piece auxiliary lever/needle chucking guide assures easy assembly. (I copied this from Dixieart)
If you can afford the extra $ for the plus then I would say do it. If for no other reason the pre-set handle will prove to be very handy.
HTH,
Mike

edit: By the way where are you getting one for half price??