Welcome Mark!
Little blue box (DS5) and cable on eBay = $150 to $250+, depending on how many people see it and want it. A rackmount (DS5R) power supply retails for $425, but call Super Bass or another big shop for discount price. |Cables new are $190 retail, but there are posts on the board for making your own, and Dela sells them once in a while on eBay for around $55 + shipping, if I remember right.
Luckiest guy in the world? Hmmm. To repair it like new (it will still not be as a new Series I), you will need a new neck = probably around $1,600. Refin, since they will not get the new finish to match the old = $1,150. Electronics upgrade, unless you live in a rural area with little RF interferance = $1,500+. Still feeling lucky? What you would have is a near-mint 77 Series I bass, which would be worth about the price you shelled out in total, but could probably get for $2,500 to $3,000 with persistence and patience, but would probably still not have the electronics upgrade.
Getting just the peghead fixed would cost you a few hundred, if you don't mind it always showing the crack. Therefore, you could have a great sounding bass for $600 to $800, with maybe some interference if you live in a city of size - if it ain't broke, don't fix it. If this would do, I agree that you are the luckiest guy around to grab the bass for only $140.
Your best bet is an estimate on the peghead repair from Mica or Valentino, and then discuss all the other possibilities with them. The 1/4 jack I believe is mono (I should know, but I never use it on either of my series basses) and I think it just outputs one of the pickups - ? What ever you decide to do, do have fun with this bass. Series basses have a great sound! Good luck Mark.