I am (unsurprisingly to those who've found my website) one of those who believes that deckplans are an important part of starship design for RPGs. I find them less useful for wargame use (unless the game is about boarding combat, of course), but a sense of internal layout is important for most other uses, including model building. Where are the airlocks, and why?
As for inspiration, I find it all over the place. I'm a long-time collector of ship pictures, because any such picture can produce multiple inspirations over time. I've also found inspiration in sea life, old issues of Popular Mechanics, and the bewildering variety of toys found on store shelves these days (as well as the packages said toys come in; often more useful than the toy in this regard).
As for drawing programs, I still find a pencil and graph-paper pad is often my best first stage, but once I get rolling a drawing program has to have certain features. A fairly quick tool for changing line color, thickness, and solidity is important to me, so that these adjustments don't interfere with the idea flow. A decent curve tool is useful as well, though being able to fake it with line segments can be a useful talent to develop.
For me, this boils down to a machine-and-OS-appropriate version of ClarisWorks at the moment, but Freehand or Canvas are good options. I use (ie. dabble in) AutoCADD for work purposes, and I find it an abomination among drafting programs, both for the purposefully obscure interface (even now) and the monopolistic and elitist business practices of the manufacturer. That said, if it floats your boat and you have it, use it.