Thanks very much for the replies!
As evidenced by my lack of prompt reply, the main barrier to scratch building is time!
I think that using some BFG ships as a basis (like Algavinn has shown) is probably not too prohibitive in cost as long as there are enough other bits available to make more ships than bought. Plasticard layering seems to be a good technique to at least build the body of an Imperial ship (with turrets, etc., added on separately).
If only I was very skilled at modelling, and able to make Eldar ships...
. I have noticed that some things from Warhammer 40K are ship-shaped, and would make cool ships. Someone also made a fleet using the new Dark Eldar, and it was very impressive! It escapes me who that was exactly, but I found the link in this forum somewhere....
Thank you!
Thinking Stone