I think that I can offer you some advice, as I held a Pro/Am license in MX, SX and WERA in my younger years. Your MX experiences will make you a natural at mtb riding and you should even enjoy the bumps, jumps and natural challenges of the trails. Mtb riding is like riding enduro bikes without the motor.

Road bike riding is very different from mtb riding (I ride both). Road rides are more linear in power output were mtb rides are more HIIT, but both will burn about the same calories per hour. I try to go out for two hours at a time.

About bikes, Trek makes some good bikes, as do many other manufacturers. Test ride a few and even try to rent some. A good dealer will let you go for a "real" ride, rather than just around the parking lot. A good bike fit is worth it's weight in gold and riding buddies help make the experience more fun. Also, their are some good deals on Craigslist for used bikes that might be a good alternative to a new bike.

Since you are familiar with dirt bikes, you already know that this is a high maintenance sport and the bikes need a lot of care and parts will break. Prevention at home is better than carrying the bike "out there" or calling for a family member to come rescue you.

Good luck with your decision and ride often.