There are law firms that will work for equity, and even some that provide a 15% discount (or similar) and relaxed payment terms for startups. So there are other viable options to equity. However, the more established firms are willing to take a chance on startups - but only if they vet them first and consider them a viable longer term company.
The 20% equity stake should be a red flag - especially for only basic legal needs. I would be very wary of this firm. Do some research - see how much it would cost you for your specific needs at other firms and go from there. Regardless of the amount, ask yourself if that is worth 20% of your company. I would suggest using the 20% for other needs - like Cofounders that can help you grow your business.