Re: OK, absolute newb here.......
Stay away from aluminum and springer forks. Aluminum frames will crack easier than steel, and springers don't have any damping. Old steel cruisers make the best MBs. Forks? There is the suntour xcm which is stronger, lighter, and has better but still not much damping than the springers. And they come with disc mounts! Or pick up a felt slant bicycle these are great for 4-stroke builds, and they have the 1-1/8 head tube so you can run any modern shock fork like a good rockshox.
It's not so much more effort to put a gas tank on any frame. Just use rubber sheet under the tank where the mounts are and you're golden.
I don't always listen to Alice in Chains..... but when I do, SO DO THE NEIGHBORS