The color head will be a pain to add to the DII (trust me on this one, I started down that road once). The mixing chamber is no issue as I just leave my 4x5 chamber in for all printing.
If it's been 40 years, start with what you have. I have trouble getting things up to snuff after a couple of months.
Best enlarging lens? As they say in racing, "Speed costs, how fast do you want to go?". ;>) Seriously though, you have little to worry about. A 16x20 is not a big enlargement from a 4x5 negative. I think you will find a 135mm easier to use than a 150mm. My 150 has been sitting quietly in its box since I got the 135.
Enjoy your new enlarger,