I think you can get a cheap humidifier for like $20, my humidity is always around 20% in the winter and it doesnt seem to make that big of a difference when starting seedlings. You can even have a fan blow over a bowl of water to raise it for a quick fix.
You should be fine, some spiders can actually be a good thing as they'll prey on other pests like fungus gnats but having no pests would be ideal of course. Just keep the grow clean and take the proper measures to prevent any sort of contamination from a nonsterile environment and it'll be one less thing to worry about.