Dh you're linking low quality high tensile bikes that are made to look pretty instead of worrying about quality. They basically exist to attract people who see people on pretty fixies and want a pretty bike too immediately.

If you want a fixed gear, then the kilo is a good option that's already been suggested. If you're really talking about once a month, twice max as far as off roading goes, then just get a road/track bike and resist the temptation to go offroad with it for a while. Then you can grab a used MTB or something on craigslist down the road and punish it. The having an extra set of x38 or x42 tires around for a wider clearance frame like that timeline around is of course an option, but I really don't see a lot of people off roading in drops either, so you're also looking at a handlebar swap

The two things you want to do with your bike are pretty different. I really think the best thing to grab a fixed gear if that's what you want and keep an eye out for a MTB / Cyclocross bike for when you get the itch to do that