I guess there's some reversed causality at work here: the Borg look like something that's grown out of an oil derrick, so it's no wonder a futuristic oil derrick would look like the Borg...
Really, what's a Trek mining vessel going to look like? Probably quite unlike a starship... Those tentacles would be very nice for hugging asteroids, for one thing.
Do we see any evidence of "self-repair" in the movie? After George Kirk broke the mining ship, it may well have stayed broken till the bitter end, two decades of repair time or not.
The movie doesn't really give Nero any time to get any Borg enhancements. In Countdown, he spends quite a few days avenging the loss of Romulus, blowing up UFP disaster relief ships and slaughtering surviving Romulan politicians, before gaining access to the Borg tech. In the movie, Spock flies to stop the supernova, does the deed (a few minutes too late), and then heads back to Vulcan, at which point Nero intercepts him. Ships in STXI are very fast, and the Jellyfish is supposed to be faster still - so Nero simply cannot afford to take the time to refit his ship in the 24th century. Given Spock's description of the events (and why would he actively lie there, even if he glosses over some details such as which star exactly did blow up?), it's a miracle Nero even has enough time to come to sufficient grips with what just happened, so that he can have his "conversation" with Robau.
Timo Saloniemi