Even after the pot-smoking kid moves out or is over 18, I suspect he'll be around, hitting Dad up for money, using drugs, staying in the basement and sleeping until 1pm, etc. Don't assume all problems will end when he turns 18/moves out.

If he moves out and Dad subsidizes the son, that's either still a dealbreaker or the price of admission. I mean if he bought a sports car that required a $800/month payment would you be as annoyed at that as at him giving Deadbeat Son $800/month? Move in and make sure your dealbreakers are known to him.

It's probably OK to stay in a LTR with him (more than fuckbuddies but less than "seriously going down the path of matrimony") as long as your kids don't really bond with him. This depends on what you -- your kids -- and he -- can all handle.

Again, tastes can vary in a number of legitimate ways, there's legit arguments to both the "introduce your kids sooner so you know if you get along" and the "wait until there's a ring and a date to introduce your kids" schools of thought.)

My Aunt got married (she had 3 kids, he had 1). Kids did not get along, so they lived separately. About a mile from each other. Kids left in a few years, but living separately worked for them. After 25 years, it looks like they are going to take the plunge and move in together...the whole family is waiting for the divorce announcement!

RUN as fast as you can and leave no forwarding address.