Being in a somewhat similar situation Diggydoes, I may be able to pass on some advice. I am a long time castle/space collector, though I did have a few of the original smaller pirate sets from my childhood (Sabre Island and smaller). My son recently got into tall ships so a spent an evening pricing out the cost of buying the Imperial Flagship (10210). Just the pieces for ship alone came up to a whopping $365 US. Now over $120 of this was for the six mast pillar pieces, who's red-brown colour was unique to that set (the incoming ship may have them as well, so that price might eventually come down from ludicrous levels). With some judicous colour subsitutions for the sake of finanacial sanity, it comes down to a more palatable $250ish before shipping, which is still quite the investment. I had been slowly collecting bits via small orders once a month as a project, and keeping to a set budget was proving a fun challenge in itself.
But, I had a huge stroke of luck last month and managed to acquire both the Black Seas Baracuda (6285) and smaller Carribean Clipper. The BSB is one hull section and one mast smaller than the IF and immensely cheaper on Bricklink. Were I to do it over, I would have simply bought the BSB (which is big already) and expanded upon it while using white sails I made myself. It would be far cheaper in the long run and I think the BSB had a better build concept for the mast/sails/rigging anyway.
Speaking of which, there are a couple of good tutorials to read in the Pirate Index: "Building Frigates" by Captain Green Hair and a few making on your own sails. They gave me a better comprehension of what I was getting into, along with a touch of inspiration.
Best of luck.