OPI is fantastic – I have access to a Sally trade card so I get them for under £5! However if you're after good high street brands, Barry M is very reasonable and of a good quality, and the Andrea Fulerton range at Superdrug have some gorgeous top coats. I've heard very good things about Model's Own nail lacquer as well but I've yet to try it. My local salon uses a lot of China Glaze alongside all the OPI, which I believe is a little bit cheaper but with quite a comprehensive range of shades.
I'd say perhaps invest in OPI's staple colours (black, red, nude, or whatever you know you'll get a lot of wear out of) and then go to Barry M for fashion shades like coral, dark green, and of course their Instant Nail Effects, which is a dupe of OPI's Black Shatter and really great quality for the money. It's also worthwhile buying a good, hard-wearing topcoat from Sally as it will prolong the life of your polish.