It's getting more difficult to get OPI polishes cheaply. Even transdesign which sells bottles that have markings on them to differentiate from regular retail are up to $6.80 each which is nearly the same price especially after the shipping costs. I used to get a lot of my OPI from Ulta when they did their buy two get one free promos & I'd combine that with a coupon. I haven't noticed that deal in a while, but of course I haven't been lurking Ulta either. The Ulta clearance section is a great place to browse but the discounts aren't always significant. I've ordered from ebay before from certain well-reviewed sellers, but you can potentially get into the world of "fakes" and whatnot. I have found the bottles I have ordered to be fine, personally, but that's a whole different topic.
As far as color inspiration, I suggest browsing nail polish blogs such as Scrangie, Vampy Varnish, All You Desire, & Alizarine Claws following it up by checking out some of the other blogs on their blogroll. There's a whole huge world out there, and you'll start finding favorite blogs to follow and get excited about upcoming collections. You'll also start quickly finding polishes you love from the swatches, and you'll get ideas of what you want.
As my collection grew, I used to decide that I wanted, say, a good "mustard yellow cream," and would search swatches until I found one that I liked. Once you start poking around the internet, you'll find that inspiration is endless.
I also suggest not limiting yourself to just OPI. Zoya is a brand I've come to really love, and if you pay attention to their website and start following blogs, you'll learn when their promotions are going on and you can snag bottles for as low as $4 each. Also, China Glaze is a great brand with a bazillion colors that are frequently on clearance at Sally's for as low as $1.50. Transdesign and head2toebeauty sell this brand for about $3 (unless it's a specialty collection), and they have some great stuff.
Also, I suggest finding a top coat and a base coat that you like and work for you to help extend the length of your manicure. I find that longevity is much more related to pigment type & finish of polish than to brand. Many blogs will tout their favorites, but I think the base coat should be related to your nail type. Good, fast drying top coats I enjoy are seche vite, poshe, & healthy hoof which are all available at Sally's and vary in viscosity and, but you will start to discover many bloggers have their die hard favorites.
Happy hunting!