I haven't conducted any simulations on this, or seen any research recommending minimum sample size per category of a covariate. Requiring 150+ respondents per category seems safe for certain, but perhaps a bit overly conservative (perhaps smaller size than 150 per category would be fine). I personally would be comfortable with n=60 per category. That' s my opinion at this point, with no firm evidence to offer.
I'd zero-center your continuous covariate, because it makes it easier to interpret the output. But, first, clean first to eliminate outliers...Winsorize (for example, set all dollar spend above the 95% percentile spend to the 95% spend amount).