Yes, I would. It doesn't hurt to price out your options. Price all the stuff to do the plywood build and price tanks too if price is really the factor. If you've never built a tank before I'd recommend doing a tiny test build with the same materials, for example build a 20gal plywood (glass, acrylic whatever your wanting to use) you can use as a frag tank or qt etc later to test your hand. When tank building everything must be straight level and accurate so this will also help you figure out what tools to aquire etc that you will need to build the large tank, as you go larger any small inaccuracies will be magnified. I'm not saying not to build your tank, I build tanks and love it but most times price is a wash untill you get to about 200gal. My custom 155gal cost more than a stock 155 but less than having someone custom build it but I built it in an L shape to wrap the corner of my room so to me it was worth it.
Looking at eBay or Craigslist dosent hurt but I'd absolutely plan on water testing etc. Are you comfortable resealing a tank? If you taking about building I'd assume resealing a tank would be an option as well and you maybe able to find something almost free if your willing to put in that kind of work.