Use either period or comma, it doesn't matter which. Use SI prefixes for 1000s. Don't use the metric 10 and 100 prefixes (centi, deci etc). Don't use a space or any other separator for 1000s, keep it simple. Spell out units other than Ω, simply because some software still has problems with UTF. Or, use it if you like - it's rarely a problem. Or skip the units altogether if it's obvious from the component. It doesn't terribly matter. The SI m means milli and nothing else. If you're concerned with UTF and reproducing µ, then use u.