I am old fat dude looking for a mtn bike style bike to ride on mostly pavement with some light off-road trail riding with my kid. Prefer a more upright riding position than a racing position. I'll take the least expensive bike that will meet my needs. No luck finding anything used around my area lately.

Since I'm old and no fun anymore, I do not care for getting muddy, dirty, etc. Is there any mtn bikes that take a full coverage set of fenders? Would this require a certain type of front fork?

To be even more uncool, I would like a rear rack. I prefer the racks that are NOT mounted on the seatpost. So, fenders and rear rack co-existing at the same time.

Does such a bike exist?

The front fender can be resolved a few different ways for almost any bike. I use a small fender that attaches to the down tube with straps, but I don't mind some debris, just try to keep it down, here's an example https://aebike.com/page.cfm?action=de...=30&SKU=FE2513. Fuller coverage can be accomplished with a fender that mounts underneath the crown of the fork, and while some forks don't cooperate, it can often be accomplished easily enough with most, here's an example https://aebike.com/page.cfm?PageID=30...ils&sku=FE4500. You might just look through aebike's fender section for a few ideas.

There are better fenders if the bike frame and fork have specific rack and fender mounts, but that is a bike these days that's more on the hybrid/commuter side of things rather than mountain biking, so you might want to look at other forum boards as well for those...good luck in your search.