My company, Lindenia, can help you with this sort of thing. Find us at lindenia.net :)
Do you happen to know of a good Accountant?
This is absolutely crucial. You want an accountant that will handle both the tax office and ZUS for you - many of them will only handle taxes and leave ZUS to you, which isn't what you want at all. Price-wise, expect to pay around 122PLN/month for a Polish speaking one in the suburbs, 180-ish a month in the centre, and around 250/month for an English speaking one. This assumes that you don't generate a lot of invoices and you just need them to handle everything for you.
Part of what Lindenia is for is to find people decent accountants - I've heard so many horror stories that I could actually weep - but unlike most of these expat services companies, we don't have any links to accountants paying us commission - our aim is to actually save you money!