Can it be written entirely by myself (never got legal training) following the examples of my competitors? If not, is it worth writing it myself and then asking a lawyer to review? Thanks!
As long as you have a limitation of liability provision, an exclusion of warranties provision, a governing law/jurisdiction provision and the right to update/amend/revise the TOS at anytime, you're probably pretty safe until such time as you can afford a lawyer to draft a proper TOS.
That said, it shouldn't cost $000s to have a lawyer prepare a TOS. I work with many startups in Canada and, unless it is a particularly novel business model or idea, I typically quote $750 (+tax) to draft the TOS. If you're getting quoted fees significantly higher than that, you should shop around.