I did ALOT of trade missions in my Sidewinder, pretty much exclusively. I upgraded it a little bit for cargo and FSD for longer jumps, if you don't choose to keep the Sidewinder when you buy a new ship the upgrades are deducted from the new purchase. When my ship was destroyed I got to pick and choose some of the upgrades that we're on the insurance cover so I elected to drop some items to save some money. I was at about 250,000 then. Eventually I saved up enough for a Cobra MKIII and that's what I have now, lore says they are produced in Lave, although I don't know if they give a discount there. I really wanted to buy other ships but I just kept saving because I felt the Cobra was reasonably priced for it's statistics. It doesn't seem as zippy as the sidewinder but I managed to upgrade it's cargo capacity to 40 today which helps massively. I just fill up with extra goods the next station needs imported aside from my trade mission goods and sell it when I get there.
Don't forget to keep enough money for insurance, I got cocky and boosted into the station entrance by accident yesterday. Insurance on my Sidewinder was about 3,600 it's 32,000+ on the Cobra and it's gas cost are higher too. Save for what you want ... the ASP looks really good, I wonder if I could have waited long enough to go from Sidewinder to Asp.