Lets see... you've admitted to being very picky about bicycles, but are going to choose a bottom of the barrel bicycle! How much fun do you think road biking is on a $300 walmart bike? Same deal off-road.
Most likely you'll be back on the road in a week or so having concluding mtb biking sucks, is too hard or etc! Same way 99% wally-world road bikers are back on the couch saying road biking sucks.
Good luck, personally i'd borrow the friends b-ride - they have offered and shouldn't ***** about a few scratch if you crash.